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A chemical element is a species of atoms having the same number of protons in their atomic nuclei (that is, the same atomic number, or Z). 118 elements are identified, of which the first 94 occur naturally on Earth with the remaining 24 being synthetic elements. There are 80 elements that have at least one stable isotope and 38 that have exclusively radionuclides, which decay over time into other elements. Iron is the most abundant element (by mass) making up Earth, while oxygen is the most common element in the Earth's crust.

Chemical elements constitute all of the ordinary matter of the universe. However astronomical observations suggest that ordinary observable matter makes up only about 15% of the matter in the universe: the remainder is dark matter; the composition of this is unknown, but it is not composed of chemical elements. The two lightest elements, hydrogen and helium, were mostly formed in the Big Bang and are the most common elements in the universe. The next three elements (lithium, beryllium and boron) were formed mostly by cosmic ray spallation, and are thus rarer than heavier elements. Formation of elements with from 6 to 26 protons occurred and continues to occur in main sequence stars via stellar nucleosynthesis. The high abundance of oxygen, silicon, and iron on Earth reflects their common production in such stars. Elements with greater than 26 protons are formed by supernova nucleosynthesis in supernovae, which, when they explode, blast these elements as supernova remnants far into space, where they may become incorporated into planets when they are formed.

The term "element" is used for atoms with a given number of protons (regardless of whether or not they are ionized or chemically bonded, e.g. hydrogen in water) as well as for a pure chemical substance consisting of a single element (e.g. hydrogen gas). For the second meaning, the terms "elementary substance" and "simple substance" have been suggested, but they have not gained much acceptance in English chemical literature, whereas in some other languages their equivalent is widely used (e.g. French corps simple, Russian простое вещество). A single element can form multiple substances differing in their structure; they are called allotropes of the element.

When different elements are chemically combined, with the atoms held together by chemical bonds, they form chemical compounds. Only a minority of elements are found uncombined as relatively pure minerals. Among the more common of such native elements are copper, silver, gold, carbon (as coal, graphite, or diamonds), and sulfur. All but a few of the most inert elements, such as noble gases and noble metals, are usually found on Earth in chemically combined form, as chemical compounds. While about 32 of the chemical elements occur on Earth in native uncombined forms, most of these occur as mixtures. For example, atmospheric air is primarily a mixture of nitrogen, oxygen, and argon, and native solid elements occur in alloys, such as that of iron and nickel.

The history of the discovery and use of the elements began with primitive human societies that found native elements like carbon, sulfur, copper and gold. Later civilizations extracted elemental copper, tin, lead and iron from their ores by smelting, using charcoal. Alchemists and chemists subsequently identified many more; all of the naturally occurring elements were known by 1950.

The properties of the chemical elements are summarized in the periodic table, which organizes the elements by increasing atomic number into rows ("periods") in which the columns ("groups") share recurring ("periodic") physical and chemical properties. Save for unstable radioactive elements with short half-lives, all of the elements are available industrially, most of them in low degrees of impurities.

Description

The lightest chemical elements are hydrogen and helium, both created by Big Bang nucleosynthesis during the first 20 minutes of the universe in a ratio of around 3:1 by mass (or 12:1 by number of atoms), along with tiny traces of the next two elements, lithium and beryllium. Almost all other elements found in nature were made by various natural methods of nucleosynthesis. On Earth, small amounts of new atoms are naturally produced in nucleogenic reactions, or in cosmogenic processes, such as cosmic ray spallation. New atoms are also naturally produced on Earth as radiogenic daughter isotopes of ongoing radioactive decay processes such as alpha decay, beta decay, spontaneous fission, cluster decay, and other rarer modes of decay.

Of the 94 naturally occurring elements, those with atomic numbers 1 through 82 each have at least one stable isotope (except for technetium, element 43 and promethium, element 61, which have no stable isotopes). Isotopes considered stable are those for which no radioactive decay has yet been observed. Elements with atomic numbers 83 through 94 are unstable to the point that radioactive decay of all isotopes can be detected. Some of these elements, notably bismuth (atomic number 83), thorium (atomic number 90), and uranium (atomic number 92), have one or more isotopes with half-lives long enough to survive as remnants of the explosive stellar nucleosynthesis that produced the heavy metals before the formation of our Solar System. At over 1.9×10 years, over a billion times longer than the current estimated age of the universe, bismuth-209 (atomic number 83) has the longest known alpha decay half-life of any naturally occurring element, and is almost always considered on par with the 80 stable elements. The very heaviest elements (those beyond plutonium, element 94) undergo radioactive decay with half-lives so short that they are not found in nature and must be synthesized.

As of 2010, there are 118 known elements (in this context, "known" means observed well enough, even from just a few decay products, to have been differentiated from other elements). Of these 118 elements, 94 occur naturally on Earth. Six of these occur in extreme trace quantities: technetium, atomic number 43; promethium, number 61; astatine, number 85; francium, number 87; neptunium, number 93; and plutonium, number 94. These 94 elements have been detected in the universe at large, in the spectra of stars and also supernovae, where short-lived radioactive elements are newly being made. The first 94 elements have been detected directly on Earth as primordial nuclides present from the formation of the solar system, or as naturally occurring fission or transmutation products of uranium and thorium.

The remaining 24 heavier elements, not found today either on Earth or in astronomical spectra, have been produced artificially: these are all radioactive, with very short half-lives; if any atoms of these elements were present at the formation of Earth, they are extremely likely, to the point of certainty, to have already decayed, and if present in novae, have been in quantities too small to have been noted. Technetium was the first purportedly non-naturally occurring element synthesized, in 1937, although trace amounts of technetium have since been found in nature (and also the element may have been discovered naturally in 1925). This pattern of artificial production and later natural discovery has been repeated with several other radioactive naturally occurring rare elements.

List of the elements are available by name, atomic number, density, melting point, boiling point and by symbol, as well as ionization energies of the elements. The nuclides of stable and radioactive elements are also available as a list of nuclides, sorted by length of half-life for those that are unstable. One of the most convenient, and certainly the most traditional presentation of the elements, is in the form of the periodic table, which groups together elements with similar chemical properties (and usually also similar electronic structures).

Atomic number

The atomic number of an element is equal to the number of protons in each atom, and defines the element. For example, all carbon atoms contain 6 protons in their atomic nucleus; so the atomic number of carbon is 6. Carbon atoms may have different numbers of neutrons; atoms of the same element having different numbers of neutrons are known as isotopes of the element.

The number of protons in the atomic nucleus also determines its electric charge, which in turn determines the number of electrons of the atom in its non-ionized state. The electrons are placed into atomic orbitals that determine the atom's various chemical properties. The number of neutrons in a nucleus usually has very little effect on an element's chemical properties (except in the case of hydrogen and deuterium). Thus, all carbon isotopes have nearly identical chemical properties because they all have six protons and six electrons, even though carbon atoms may, for example, have 6 or 8 neutrons. That is why the atomic number, rather than mass number or atomic weight, is considered the identifying characteristic of a chemical element.

The symbol for atomic number is Z.

Isotopes

Isotopes are atoms of the same element (that is, with the same number of protons in their atomic nucleus), but having different numbers of neutrons. Thus, for example, there are three main isotopes of carbon. All carbon atoms have 6 protons in the nucleus, but they can have either 6, 7, or 8 neutrons. Since the mass numbers of these are 12, 13 and 14 respectively, the three isotopes of carbon are known as carbon-12, carbon-13, and carbon-14, often abbreviated to C, C, and C. Carbon in everyday life and in chemistry is a mixture of C (about 98.9%), C (about 1.1%) and about 1 atom per trillion of C.

Most (66 of 94) naturally occurring elements have more than one stable isotope. Except for the isotopes of hydrogen (which differ greatly from each other in relative mass—enough to cause chemical effects), the isotopes of a given element are chemically nearly indistinguishable.

All of the elements have some isotopes that are radioactive (radioisotopes), although not all of these radioisotopes occur naturally. The radioisotopes typically decay into other elements upon radiating an alpha or beta particle. If an element has isotopes that are not radioactive, these are termed "stable" isotopes. All of the known stable isotopes occur naturally (see primordial isotope). The many radioisotopes that are not found in nature have been characterized after being artificially made. Certain elements have no stable isotopes and are composed only of radioactive isotopes: specifically the elements without any stable isotopes are technetium (atomic number 43), promethium (atomic number 61), and all observed elements with atomic numbers greater than 82.

Of the 80 elements with at least one stable isotope, 26 have only one single stable isotope. The mean number of stable isotopes for the 80 stable elements is 3.1 stable isotopes per element. The largest number of stable isotopes that occur for a single element is 10 (for tin, element 50).

Isotopic mass and atomic mass

The mass number of an element, A, is the number of nucleons (protons and neutrons) in the atomic nucleus. Different isotopes of a given element are distinguished by their mass numbers, which are conventionally written as a superscript on the left hand side of the atomic symbol (e.g. U). The mass number is always a whole number and has units of "nucleons". For example, magnesium-24 (24 is the mass number) is an atom with 24 nucleons (12 protons and 12 neutrons).

Whereas the mass number simply counts the total number of neutrons and protons and is thus a natural (or whole) number, the atomic mass of a single atom is a real number giving the mass of a particular isotope (or "nuclide") of the element, expressed in atomic mass units (symbol: u). In general, the mass number of a given nuclide differs in value slightly from its atomic mass, since the mass of each proton and neutron is not exactly 1 u; since the electrons contribute a lesser share to the atomic mass as neutron number exceeds proton number; and (finally) because of the nuclear binding energy. For example, the atomic mass of chlorine-35 to five significant digits is 34.969 u and that of chlorine-37 is 36.966 u. However, the atomic mass in u of each isotope is quite close to its simple mass number (always within 1%). The only isotope whose atomic mass is exactly a natural number is C, which by definition has a mass of exactly 12, because u is defined as 1/12 of the mass of a free neutral carbon-12 atom in the ground state.

The standard atomic weight (commonly called "atomic weight") of an element is the average of the atomic masses of all the chemical element's isotopes as found in a particular environment, weighted by isotopic abundance, relative to the atomic mass unit. This number may be a fraction that is not close to a whole number. For example, the relative atomic mass of chlorine is 35.453 u, which differs greatly from a whole number as it is an average of about 76% chlorine-35 and 24% chlorine-37. Whenever a relative atomic mass value differs by more than 1% from a whole number, it is due to this averaging effect, as significant amounts of more than one isotope are naturally present in a sample of that element.

Chemically pure and isotopically pure

Chemists and nuclear scientists have different definitions of a pure element. In chemistry, a pure element means a substance whose atoms all (or in practice almost all) have the same atomic number, or number of protons. Nuclear scientists, however, define a pure element as one that consists of only one stable isotope.

For example, a copper wire is 99.99% chemically pure if 99.99% of its atoms are copper, with 29 protons each. However it is not isotopically pure since ordinary copper consists of two stable isotopes, 69% Cu and 31% Cu, with different numbers of neutrons. However, a pure gold ingot would be both chemically and isotopically pure, since ordinary gold consists only of one isotope, Au.

Allotropes

Atoms of chemically pure elements may bond to each other chemically in more than one way, allowing the pure element to exist in multiple chemical structures (spatial arrangements of atoms), known as allotropes, which differ in their properties. For example, carbon can be found as diamond, which has a tetrahedral structure around each carbon atom; graphite, which has layers of carbon atoms with a hexagonal structure stacked on top of each other; graphene, which is a single layer of graphite that is very strong; fullerenes, which have nearly spherical shapes; and carbon nanotubes, which are tubes with a hexagonal structure (even these may differ from each other in electrical properties). The ability of an element to exist in one of many structural forms is known as 'allotropy'.

The standard state, also known as reference state, of an element is defined as its thermodynamically most stable state at a pressure of 1 bar and a given temperature (typically at 298.15 K). In thermochemistry, an element is defined to have an enthalpy of formation of zero in its standard state. For example, the reference state for carbon is graphite, because the structure of graphite is more stable than that of the other allotropes.

Properties

Several kinds of descriptive categorizations can be applied broadly to the elements, including consideration of their general physical and chemical properties, their states of matter under familiar conditions, their melting and boiling points, their densities, their crystal structures as solids, and their origins.

General properties

Several terms are commonly used to characterize the general physical and chemical properties of the chemical elements. A first distinction is between metals, which readily conduct electricity, nonmetals, which do not, and a small group, (the metalloids), having intermediate properties and often behaving as semiconductors.

A more refined classification is often shown in colored presentations of the periodic table. This system restricts the terms "metal" and "nonmetal" to only certain of the more broadly defined metals and nonmetals, adding additional terms for certain sets of the more broadly viewed metals and nonmetals. The version of this classification used in the periodic tables presented here includes: actinides, alkali metals, alkaline earth metals, halogens, lanthanides, transition metals, post-transition metals, metalloids, reactive nonmetals, and noble gases. In this system, the alkali metals, alkaline earth metals, and transition metals, as well as the lanthanides and the actinides, are special groups of the metals viewed in a broader sense. Similarly, the reactive nonmetals and the noble gases are nonmetals viewed in the broader sense. In some presentations, the halogens are not distinguished, with astatine identified as a metalloid and the others identified as nonmetals.

States of matter

Another commonly used basic distinction among the elements is their state of matter (phase), whether solid, liquid, or gas, at a selected standard temperature and pressure (STP). Most of the elements are solids at conventional temperatures and atmospheric pressure, while several are gases. Only bromine and mercury are liquids at 0 degrees Celsius (32 degrees Fahrenheit) and normal atmospheric pressure; caesium and gallium are solids at that temperature, but melt at 28.4 °C (83.2 °F) and 29.8 °C (85.6 °F), respectively.

Melting and boiling points

Melting and boiling points, typically expressed in degrees Celsius at a pressure of one atmosphere, are commonly used in characterizing the various elements. While known for most elements, either or both of these measurements is still undetermined for some of the radioactive elements available in only tiny quantities. Since helium remains a liquid even at absolute zero at atmospheric pressure, it has only a boiling point, and not a melting point, in conventional presentations.

Densities

The density at a selected standard temperature and pressure (STP) is frequently used in characterizing the elements. Density is often expressed in grams per cubic centimeter (g/cm). Since several elements are gases at commonly encountered temperatures, their densities are usually stated for their gaseous forms; when liquefied or solidified, the gaseous elements have densities similar to those of the other elements.

When an element has allotropes with different densities, one representative allotrope is typically selected in summary presentations, while densities for each allotrope can be stated where more detail is provided. For example, the three familiar allotropes of carbon (amorphous carbon, graphite, and diamond) have densities of 1.8–2.1, 2.267, and 3.515 g/cm, respectively.

Crystal structures

The elements studied to date as solid samples have eight kinds of crystal structures: cubic, body-centered cubic, face-centered cubic, hexagonal, monoclinic, orthorhombic, rhombohedral, and tetragonal. For some of the synthetically produced transuranic elements, available samples have been too small to determine crystal structures.

Occurrence and origin on Earth

Chemical elements may also be categorized by their origin on Earth, with the first 94 considered naturally occurring, while those with atomic numbers beyond 94 have only been produced artificially as the synthetic products of man-made nuclear reactions.

Of the 94 naturally occurring elements, 83 are considered primordial and either stable or weakly radioactive. The remaining 11 naturally occurring elements possess half lives too short for them to have been present at the beginning of the Solar System, and are therefore considered transient elements. Of these 11 transient elements, 5 (polonium, radon, radium, actinium, and protactinium) are relatively common decay products of thorium and uranium. The remaining 6 transient elements (technetium, promethium, astatine, francium, neptunium, and plutonium) occur only rarely, as products of rare decay modes or nuclear reaction processes involving uranium or other heavy elements.

Elements with atomic numbers 1 through 40 are all stable, while those with atomic numbers 41 through 82 (except technetium and promethium) are metastable. The half-lives of these metastable "theoretical radionuclides" are so long (at least 100 million times longer than the estimated age of the universe) that their radioactive decay has yet to be detected by experiment. Elements with atomic numbers 83 through 94 are unstable to the point that their radioactive decay can be detected. Three of these elements, bismuth (element 83), thorium (element 90), and uranium (element 92) have one or more isotopes with half-lives long enough to survive as remnants of the explosive stellar nucleosynthesis that produced the heavy elements before the formation of our solar system. For example, at over 1.9×10 years, over a billion times longer than the current estimated age of the universe, bismuth-209 has the longest known alpha decayhalf-life of any naturally occurring element. The very heaviest 24 elements (those beyond plutonium, element 94) undergo radioactive decay with short half-lives and cannot be produced as daughters of longer-lived elements, and thus they do not occur in nature at all.

The periodic table

Periodic table

Group	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18
	Alkali metals	Alkaline earth metals													Pnictogens	Chalcogens	Halogens	Noble gases
Period 1	Hydrogen 1 H 1.008																	Helium 2 He 4.0026
2	Lithium 3 Li 6.94	Beryllium 4 Be 9.0122											Boron 5 B 10.81	Carbon 6 C 12.011	Nitrogen 7 N 14.007	Oxygen 8 O 15.999	Fluorine 9 F 18.998	Neon 10 Ne 20.180
3	Sodium 11 Na 22.990	Magnesium 12 Mg 24.305											Aluminium 13 Al 26.982	Silicon 14 Si 28.085	Phosphorus 15 P 30.974	Sulfur 16 S 32.06	Chlorine 17 Cl 35.45	Argon 18 Ar 39.948
4	Potassium 19 K 39.098	Calcium 20 Ca 40.078	Scandium 21 Sc 44.956	Titanium 22 Ti 47.867	Vanadium 23 V 50.942	Chromium 24 Cr 51.996	Manganese 25 Mn 54.938	Iron 26 Fe 55.845	Cobalt 27 Co 58.933	Nickel 28 Ni 58.693	Copper 29 Cu 63.546	Zinc 30 Zn 65.38	Gallium 31 Ga 69.723	Germanium 32 Ge 72.630	Arsenic 33 As 74.922	Selenium 34 Se 78.971	Bromine 35 Br 79.904	Krypton 36 Kr 83.798
5	Rubidium 37 Rb 85.468	Strontium 38 Sr 87.62	Yttrium 39 Y 88.906	Zirconium 40 Zr 91.224	Niobium 41 Nb 92.906	Molybdenum 42 Mo 95.95	Technetium 43 Tc [98]	Ruthenium 44 Ru 101.07	Rhodium 45 Rh 102.91	Palladium 46 Pd 106.42	Silver 47 Ag 107.87	Cadmium 48 Cd 112.41	Indium 49 In 114.82	Tin 50 Sn 118.71	Antimony 51 Sb 121.76	Tellurium 52 Te 127.60	Iodine 53 I 126.90	Xenon 54 Xe 131.29
6	Caesium 55 Cs 132.91	Barium 56 Ba 137.33	Lanthanum 57 La 138.91	Hafnium 72 Hf 178.49	Tantalum 73 Ta 180.95	Tungsten 74 W 183.84	Rhenium 75 Re 186.21	Osmium 76 Os 190.23	Iridium 77 Ir 192.22	Platinum 78 Pt 195.08	Gold 79 Au 196.97	Mercury 80 Hg 200.59	Thallium 81 Tl 204.38	Lead 82 Pb 207.2	Bismuth 83 Bi 208.98	Polonium 84 Po [209]	Astatine 85 At [210]	Radon 86 Rn [222]
7	Francium 87 Fr [223]	Radium 88 Ra [226]	Actinium 89 Ac [227]	Rutherfordium 104 Rf [267]	Dubnium 105 Db [268]	Seaborgium 106 Sg [269]	Bohrium 107 Bh [270]	Hassium 108 Hs [270]	Meitnerium 109 Mt [278]	Darmstadtium 110 Ds [281]	Roentgenium 111 Rg [282]	Copernicium 112 Cn [285]	Nihonium 113 Nh [286]	Flerovium 114 Fl [289]	Moscovium 115 Mc [290]	Livermorium 116 Lv [293]	Tennessine 117 Ts [294]	Oganesson 118 Og [294]

				Cerium 58 Ce 140.12	Praseodymium 59 Pr 140.91	Neodymium 60 Nd 144.24	Promethium 61 Pm [145]	Samarium 62 Sm 150.36	Europium 63 Eu 151.96	Gadolinium 64 Gd 157.25	Terbium 65 Tb 158.93	Dysprosium 66 Dy 162.50	Holmium 67 Ho 164.93	Erbium 68 Er 167.26	Thulium 69 Tm 168.93	Ytterbium 70 Yb 173.05	Lutetium 71 Lu 174.97
				Thorium 90 Th 232.04	Protactinium 91 Pa 231.04	Uranium 92 U 238.03	Neptunium 93 Np [237]	Plutonium 94 Pu [244]	Americium 95 Am [243]	Curium 96 Cm [247]	Berkelium 97 Bk [247]	Californium 98 Cf [251]	Einsteinium 99 Es [252]	Fermium 100 Fm [257]	Mendelevium 101 Md [258]	Nobelium 102 No [259]	Lawrencium 103 Lr [266]

1 (red)=Gas 3 (black)=Solid 80 (green)=Liquid 109 (gray)=Unknown Color of the atomic number shows state of matter (at 0 °C and 1 atm)

Primordial
From decay Synthetic Border shows natural occurrence of the element

Standard atomic weight (A_r)

Ca: 40.078 — Formal short value, rounded (no uncertainty)
Po: [209] — mass number of the most stable isotope

Background color shows subcategory in the metal–metalloid–nonmetal trend:

Metal						Metalloid	Nonmetal			Unknown chemical properties
Alkali metal	Alkaline earth metal	Lanthanide	Actinide	Transition metal	Post-transition metal		Reactive nonmetal	Noble gas

The properties of the chemical elements are often summarized using the periodic table, which powerfully and elegantly organizes the elements by increasing atomic number into rows ("periods") in which the columns ("groups") share recurring ("periodic") physical and chemical properties. The current standard table contains 118 confirmed elements as of 10 April 2010.

Although earlier precursors to this presentation exist, its invention is generally credited to the Russian chemist Dmitri Mendeleev in 1869, who intended the table to illustrate recurring trends in the properties of the elements. The layout of the table has been refined and extended over time as new elements have been discovered and new theoretical models have been developed to explain chemical behavior.

Use of the periodic table is now ubiquitous within the academic discipline of chemistry, providing an extremely useful framework to classify, systematize and compare all the many different forms of chemical behavior. The table has also found wide application in physics, geology, biology, materials science, engineering, agriculture, medicine, nutrition, environmental health, and astronomy. Its principles are especially important in chemical engineering.

Nomenclature and symbols

The various chemical elements are formally identified by their unique atomic numbers, by their accepted names, and by their symbols.

Atomic numbers

The known elements have atomic numbers from 1 through 118, conventionally presented as Arabic numerals. Since the elements can be uniquely sequenced by atomic number, conventionally from lowest to highest (as in a periodic table), sets of elements are sometimes specified by such notation as "through", "beyond", or "from... through", as in "through iron", "beyond uranium", or "from lanthanum through lutetium". The terms "light" and "heavy" are sometimes also used informally to indicate relative atomic numbers (not densities), as in "lighter than carbon" or "heavier than lead", although technically the weight or mass of atoms of an element (their atomic weights or atomic masses) do not always increase monotonically with their atomic numbers.

Element names

The naming of various substances now known as elements precedes the atomic theory of matter, as names were given locally by various cultures to various minerals, metals, compounds, alloys, mixtures, and other materials, although at the time it was not known which chemicals were elements and which compounds. As they were identified as elements, the existing names for anciently-known elements (e.g., gold, mercury, iron) were kept in most countries. National differences emerged over the names of elements either for convenience, linguistic niceties, or nationalism. For a few illustrative examples: German speakers use "Wasserstoff" (water substance) for "hydrogen", "Sauerstoff" (acid substance) for "oxygen" and "Stickstoff" (smothering substance) for "nitrogen", while English and some romance languages use "sodium" for "natrium" and "potassium" for "kalium", and the French, Italians, Greeks, Portuguese and Poles prefer "azote/azot/azoto" (from roots meaning "no life") for "nitrogen".

For purposes of international communication and trade, the official names of the chemical elements both ancient and more recently recognized are decided by the International Union of Pure and Applied Chemistry (IUPAC), which has decided on a sort of international English language, drawing on traditional English names even when an element's chemical symbol is based on a Latin or other traditional word, for example adopting "gold" rather than "aurum" as the name for the 79th element (Au). IUPAC prefers the British spellings "aluminium" and "caesium" over the U.S. spellings "aluminum" and "cesium", and the U.S. "sulfur" over the British "sulphur". However, elements that are practical to sell in bulk in many countries often still have locally used national names, and countries whose national language does not use the Latin alphabet are likely to use the IUPAC element names.

According to IUPAC, chemical elements are not proper nouns in English; consequently, the full name of an element is not routinely capitalized in English, even if derived from a proper noun, as in californium and einsteinium. Isotope names of chemical elements are also uncapitalized if written out, e.g., carbon-12 or uranium-235. Chemical element symbols (such as Cf for californium and Es for einsteinium), are always capitalized (see below).

In the second half of the twentieth century, physics laboratories became able to produce nuclei of chemical elements with half-lives too short for an appreciable amount of them to exist at any time. These are also named by IUPAC, which generally adopts the name chosen by the discoverer. This practice can lead to the controversial question of which research group actually discovered an element, a question that delayed the naming of elements with atomic number of 104 and higher for a considerable amount of time. (See element naming controversy).

Precursors of such controversies involved the nationalistic namings of elements in the late 19th century. For example, lutetium was named in reference to Paris, France. The Germans were reluctant to relinquish naming rights to the French, often calling it cassiopeium. Similarly, the British discoverer of niobium originally named it columbium, in reference to the New World. It was used extensively as such by American publications prior to the international standardization (in 1950).

Chemical symbols

Specific chemical elements

Before chemistry became a science, alchemists had designed arcane symbols for both metals and common compounds. These were however used as abbreviations in diagrams or procedures; there was no concept of atoms combining to form molecules. With his advances in the atomic theory of matter, John Dalton devised his own simpler symbols, based on circles, to depict molecules.

The current system of chemical notation was invented by Berzelius. In this typographical system, chemical symbols are not mere abbreviations—though each consists of letters of the Latin alphabet. They are intended as universal symbols for people of all languages and alphabets.

The first of these symbols were intended to be fully universal. Since Latin was the common language of science at that time, they were abbreviations based on the Latin names of metals. Cu comes from Cuprum, Fe comes from Ferrum, Ag from Argentum. The symbols were not followed by a period (full stop) as with abbreviations. Later chemical elements were also assigned unique chemical symbols, based on the name of the element, but not necessarily in English. For example, sodium has the chemical symbol 'Na' after the Latin natrium. The same applies to "W" (wolfram) for tungsten, "Fe" (ferrum) for iron, "Hg" (hydrargyrum) for mercury, "Sn" (stannum) for tin, "K" (kalium) for potassium, "Au" (aurum) for gold, "Ag" (argentum) for silver, "Pb" (plumbum) for lead, "Cu" (cuprum) for copper, and "Sb" (stibium) for antimony.

Chemical symbols are understood internationally when element names might require translation. There have sometimes been differences in the past. For example, Germans in the past have used "J" (for the alternate name Jod) for iodine, but now use "I" and "Iod".

The first letter of a chemical symbol is always capitalized, as in the preceding examples, and the subsequent letters, if any, are always lower case (small letters). Thus, the symbols for californium and einsteinium are Cf and Es.

General chemical symbols

There are also symbols in chemical equations for groups of chemical elements, for example in comparative formulas. These are often a single capital letter, and the letters are reserved and not used for names of specific elements. For example, an "X" indicates a variable group (usually a halogen) in a class of compounds, while "R" is a radical, meaning a compound structure such as a hydrocarbon chain. The letter "Q" is reserved for "heat" in a chemical reaction. "Y" is also often used as a general chemical symbol, although it is also the symbol of yttrium. "Z" is also frequently used as a general variable group. "E" is used in organic chemistry to denote an electron-withdrawing group or an electrophile; similarly "Nu" denotes a nucleophile. "L" is used to represent a general ligand in inorganic and organometallic chemistry. "M" is also often used in place of a general metal.

At least two additional, two-letter generic chemical symbols are also in informal usage, "Ln" for any lanthanide element and "An" for any actinide element. "Rg" was formerly used for any rare gas element, but the group of rare gases has now been renamed noble gases and the symbol "Rg" has now been assigned to the element roentgenium.

Isotope symbols

Isotopes are distinguished by the atomic mass number (total protons and neutrons) for a particular isotope of an element, with this number combined with the pertinent element's symbol. IUPAC prefers that isotope symbols be written in superscript notation when practical, for example C and U. However, other notations, such as carbon-12 and uranium-235, or C-12 and U-235, are also used.

As a special case, the three naturally occurring isotopes of the element hydrogen are often specified as H for H (protium), D for H (deuterium), and T for H (tritium). This convention is easier to use in chemical equations, replacing the need to write out the mass number for each atom. For example, the formula for heavy water may be written D₂O instead of H₂O.

Origin of the elements

Estimated distribution of dark matter and dark energy in the universe. Only the fraction of the mass and energy in the universe labeled "atoms" is composed of chemical elements.

Only about 4% of the total mass of the universe is made of atoms or ions, and thus represented by chemical elements. This fraction is about 15% of the total matter, with the remainder of the matter (85%) being dark matter. The nature of dark matter is unknown, but it is not composed of atoms of chemical elements because it contains no protons, neutrons, or electrons. (The remaining non-matter part of the mass of the universe is composed of the even more mysterious dark energy).

The universe's 94 naturally occurring chemical elements are thought to have been produced by at least four cosmic processes. Most of the hydrogen, helium and a very small quantity of lithiumin the universe was produced primordially in the first few minutes of the Big Bang. Other three recurrently occurring later processes are thought to have produced the remaining elements. Stellar nucleosynthesis, an ongoing process inside stars, produces all elements from carbon through iron in atomic number, but little lithium, beryllium, or boron. Elements heavier in atomic number than iron, as heavy as uranium and plutonium, are produced by explosive nucleosynthesis in supernovas and other cataclysmic cosmic events. Cosmic ray spallation(fragmentation) of carbon, nitrogen, and oxygen is important to the production of lithium, beryllium and boron.

During the early phases of the Big Bang, nucleosynthesis of hydrogen nuclei resulted in the production of hydrogen-1 (protium, H) and helium-4 (He), as well as a smaller amount of deuterium (H) and very minuscule amounts (on the order of 10) of lithium and beryllium. Even smaller amounts of boron may have been produced in the Big Bang, since it has been observed in some very old stars, while carbon has not. It is generally agreed that no heavier elements than boron were produced in the Big Bang. As a result, the primordial abundance of atoms (or ions) consisted of roughly 75% H, 25% He, and 0.01% deuterium, with only tiny traces of lithium, beryllium, and perhaps boron. Subsequent enrichment of galactic halos occurred due to stellar nucleosynthesis and supernova nucleosynthesis. However, the element abundance in intergalactic space can still closely resemble primordial conditions, unless it has been enriched by some means.

Periodic table showing the cosmogenic origin of each element in the Big Bang, or in large or small stars. Small stars can produce certain elements up to sulfur, by the alpha process. Supernovae are needed to produce "heavy" elements (those beyond iron and nickel) rapidly by neutron buildup, in the r-process. Certain large stars slowly produce other elements heavier than iron, in the s-process; these may then be blown into space in the off-gassing of planetary nebulae

On Earth (and elsewhere), trace amounts of various elements continue to be produced from other elements as products of nuclear transmutation processes. These include some produced by cosmic rays or other nuclear reactions (see cosmogenic and nucleogenic nuclides), and others produced as decay products of long-lived primordial nuclides. For example, trace (but detectable) amounts of carbon-14 (C) are continually produced in the atmosphere by cosmic rays impacting nitrogen atoms, and argon-40 (Ar) is continually produced by the decay of primordially occurring but unstable potassium-40 (K). Also, three primordially occurring but radioactive actinides, thorium, uranium, and plutonium, decay through a series of recurrently produced but unstable radioactive elements such as radium and radon, which are transiently present in any sample of these metals or their ores or compounds. Three other radioactive elements, technetium, promethium, and neptunium, occur only incidentally in natural materials, produced as individual atoms by nuclear fission of the nuclei of various heavy elements or in other rare nuclear processes.

Human technology has produced various additional elements beyond these first 94, with those through atomic number 118 now known.

Abundance

The following graph (note log scale) shows the abundance of elements in our Solar System. The table shows the twelve most common elements in our galaxy (estimated spectroscopically), as measured in parts per million, by mass. Nearby galaxies that have evolved along similar lines have a corresponding enrichment of elements heavier than hydrogen and helium. The more distant galaxies are being viewed as they appeared in the past, so their abundances of elements appear closer to the primordial mixture. As physical laws and processes appear common throughout the visible universe, however, scientist expect that these galaxies evolved elements in similar abundance.

The abundance of elements in the Solar System is in keeping with their origin from nucleosynthesis in the Big Bang and a number of progenitor supernova stars. Very abundant hydrogen and helium are products of the Big Bang, but the next three elements are rare since they had little time to form in the Big Bang and are not made in stars (they are, however, produced in small quantities by the breakup of heavier elements in interstellar dust, as a result of impact by cosmic rays). Beginning with carbon, elements are produced in stars by buildup from alpha particles (helium nuclei), resulting in an alternatingly larger abundance of elements with even atomic numbers (these are also more stable). In general, such elements up to iron are made in large stars in the process of becoming supernovas. Iron-56 is particularly common, since it is the most stable element that can easily be made from alpha particles (being a product of decay of radioactive nickel-56, ultimately made from 14 helium nuclei). Elements heavier than iron are made in energy-absorbing processes in large stars, and their abundance in the universe (and on Earth) generally decreases with their atomic number.

The abundance of the chemical elements on Earth varies from air to crust to ocean, and in various types of life. The abundance of elements in Earth's crust differs from that in the Solar system (as seen in the Sun and heavy planets like Jupiter) mainly in selective loss of the very lightest elements (hydrogen and helium) and also volatile neon, carbon (as hydrocarbons), nitrogen and sulfur, as a result of solar heating in the early formation of the solar system. Oxygen, the most abundant Earth element by mass, is retained on Earth by combination with silicon. Aluminum at 8% by mass is more common in the Earth's crust than in the universe and solar system, but the composition of the far more bulky mantle, which has magnesium and iron in place of aluminum (which occurs there only at 2% of mass) more closely mirrors the elemental composition of the solar system, save for the noted loss of volatile elements to space, and loss of iron which has migrated to the Earth's core.

The composition of the human body, by contrast, more closely follows the composition of seawater—save that the human body has additional stores of carbon and nitrogen necessary to form the proteins and nucleic acids, together with phosphorus in the nucleic acids and energy transfer molecule adenosine triphosphate (ATP) that occurs in the cells of all living organisms. Certain kinds of organisms require particular additional elements, for example the magnesium in chlorophyll in green plants, the calcium in mollusc shells, or the iron in the hemoglobin in vertebrate animals' red blood cells.

Abundances of the chemical elements in the Solar system. Hydrogen and helium are most common, from the Big Bang. The next three elements (Li, Be, B) are rare because they are poorly synthesized in the Big Bang and also in stars. The two general trends in the remaining stellar-produced elements are: (1) an alternation of abundance in elements as they have even or odd atomic numbers (the Oddo-Harkins rule), and (2) a general decrease in abundance as elements become heavier. Iron is especially common because it represents the minimum energy nuclide that can be made by fusion of helium in supernovae.

Elements in our galaxy	Parts per million by mass
Hydrogen	739,000
Helium	240,000
Oxygen	10,400
Carbon	4,600
Neon	1,340
Iron	1,090
Nitrogen	960
Silicon	650
Magnesium	580
Sulfur	440
Potassium	210
Nickel	100

**Nutritional elements in the periodic table**
H																		He
Li	Be												B	C	N	O	F	Ne
Na	Mg												Al	Si	P	S	Cl	Ar
K	Ca	Sc		Ti	V	Cr	Mn	Fe	Co	Ni	Cu	Zn	Ga	Ge	As	Se	Br	Kr
Rb	Sr	Y		Zr	Nb	Mo	Tc	Ru	Rh	Pd	Ag	Cd	In	Sn	Sb	Te	I	Xe
Cs	Ba	La	*	Hf	Ta	W	Re	Os	Ir	Pt	Au	Hg	Tl	Pb	Bi	Po	At	Rn
Fr	Ra	Ac	**	Rf	Db	Sg	Bh	Hs	Mt	Ds	Rg	Cn	Nh	Fl	Mc	Lv	Ts	Og

			*	Ce	Pr	Nd	Pm	Sm	Eu	Gd	Tb	Dy	Ho	Er	Tm	Yb	Lu
			**	Th	Pa	U	Np	Pu	Am	Cm	Bk	Cf	Es	Fm	Md	No	Lr

The four basic organic elements

Quantity elements

Essential trace elements

Deemed essential trace element by U.S., not by European Union

Suggested function from deprivation effects or active metabolic handling, but no clearly-identified biochemical function in humans

Limited circumstantial evidence for trace benefits or biological action in mammals

No evidence for biological action in mammals, but essential in some lower organisms.
(In the case of lanthanum, the definition of an essential nutrient as being indispensable and irreplaceable is not completely applicable due to the extreme similarity of the lanthanides. Thus Ce, Pr, and Nd may be substituted for La without ill effects for organisms using La, and the smaller Sm, Eu, and Gd may also be similarly substituted but cause slower growth.)

History

Mendeleev's 1869 periodic table: An experiment on a system of elements. Based on their atomic weights and chemical similarities.

Evolving definitions

The concept of an "element" as an undivisible substance has developed through three major historical phases: Classical definitions (such as those of the ancient Greeks), chemical definitions, and atomic definitions.

Classical definitions

Ancient philosophy posited a set of classical elements to explain observed patterns in nature. These elements originally referred to earth, water, air and fire rather than the chemical elements of modern science.

The term 'elements' (stoicheia) was first used by the Greek philosopher Plato in about 360 BCE in his dialogue Timaeus, which includes a discussion of the composition of inorganic and organic bodies and is a speculative treatise on chemistry. Plato believed the elements introduced a century earlier by Empedocles were composed of small polyhedral forms: tetrahedron (fire), octahedron (air), icosahedron(water), and cube (earth).

Aristotle, c. 350 BCE, also used the term stoicheia and added a fifth element called aether, which formed the heavens. Aristotle defined an element as:

Element – one of those bodies into which other bodies can decompose, and that itself is not capable of being divided into other.

Chemical definitions

In 1661, Robert Boyle proposed his theory of corpuscularism which favoured the analysis of matter as constituted by irreducible units of matter (atoms) and, choosing to side with neither Aristotle's view of the four elements nor Paracelsus' view of three fundamental elements, left open the question of the number of elements. The first modern list of chemical elements was given in Antoine Lavoisier's 1789 Elements of Chemistry, which contained thirty-three elements, including light and caloric. By 1818, Jöns Jakob Berzelius had determined atomic weights for forty-five of the forty-nine then-accepted elements. Dmitri Mendeleev had sixty-six elements in his periodic table of 1869.

Dmitri Mendeleev

From Boyle until the early 20th century, an element was defined as a pure substance that could not be decomposed into any simpler substance. Put another way, a chemical element cannot be transformed into other chemical elements by chemical processes. Elements during this time were generally distinguished by their atomic weights, a property measurable with fair accuracy by available analytical techniques.

Atomic definitions

Henry Moseley

The 1913 discovery by English physicist Henry Moseley that the nuclear charge is the physical basis for an atom's atomic number, further refined when the nature of protons and neutrons became appreciated, eventually led to the current definition of an element based on atomic number (number of protons per atomic nucleus). The use of atomic numbers, rather than atomic weights, to distinguish elements has greater predictive value (since these numbers are integers), and also resolves some ambiguities in the chemistry-based view due to varying properties of isotopes and allotropes within the same element. Currently, IUPAC defines an element to exist if it has isotopes with a lifetime longer than the 10 seconds it takes the nucleus to form an electronic cloud.

By 1914, seventy-two elements were known, all naturally occurring. The remaining naturally occurring elements were discovered or isolated in subsequent decades, and various additional elements have also been produced synthetically, with much of that work pioneered by Glenn T. Seaborg. In 1955, element 101 was discovered and named mendelevium in honor of D.I. Mendeleev, the first to arrange the elements in a periodic manner. Most recently, the synthesis of element 118 was reported in October 2006, and the synthesis of element 117 was reported in April 2010.

Discovery and recognition of various elements

Ten materials familiar to various prehistoric cultures are now known to be chemical elements: Carbon, copper, gold, iron, lead, mercury, silver, sulfur, tin, and zinc. Three additional materials now accepted as elements, arsenic, antimony, and bismuth, were recognized as distinct substances prior to 1500 AD. Phosphorus, cobalt, and platinum were isolated before 1750.

Most of the remaining naturally occurring chemical elements were identified and characterized by 1900, including:

Such now-familiar industrial materials as aluminium, silicon, nickel, chromium, magnesium, and tungsten
Reactive metals such as lithium, sodium, potassium, and calcium
The halogens fluorine, chlorine, bromine, and iodine
Gases such as hydrogen, oxygen, nitrogen, helium, argon, and neon
Most of the rare-earth elements, including cerium, lanthanum, gadolinium, and neodymium.
The more common radioactive elements, including uranium, thorium, radium, and radon

Elements isolated or produced since 1900 include:

The three remaining undiscovered regularly occurring stable natural elements: hafnium, lutetium, and rhenium
Plutonium, which was first produced synthetically in 1940 by Glenn T. Seaborg, but is now also known from a few long-persisting natural occurrences
The three incidentally occurring natural elements (neptunium, promethium, and technetium), which were all first produced synthetically but later discovered in trace amounts in certain geological samples
Three scarce decay products of uranium or thorium, (astatine, francium, and protactinium), and
Various synthetic transuranic elements, beginning with americium and curium

Recently discovered elements

The first transuranium element (element with atomic number greater than 92) discovered was neptunium in 1940. Since 1999 claims for the discovery of new elements have been considered by the IUPAC/IUPAP Joint Working Party. As of January 2016, all 118 elements have been confirmed as discovered by IUPAC. The discovery of element 112 was acknowledged in 2009, and the name copernicium and the atomic symbol Cn were suggested for it. The name and symbol were officially endorsed by IUPAC on 19 February 2010. The heaviest element that is believed to have been synthesized to date is element 118, oganesson, on 9 October 2006, by the Flerov Laboratory of Nuclear Reactions in Dubna, Russia. Tennessine, element 117 was the latest element claimed to be discovered, in 2009. On 28 November 2016, scientists at the IUPAC officially recognized the names for four of the newest chemical elements, with atomic numbers 113, 115, 117, and 118.

List of the 118 known chemical elements

The following sortable table shows the 118 known chemical elements.

Atomic number, name, and symbol all serve independently as unique identifiers.
Names are those accepted by IUPAC; provisional names for recently produced elements not yet formally named are in parentheses.
Group, period, and block refer to an element's position in the periodic table. Group numbers here show the currently accepted numbering; for older alternate numberings, see Group (periodic table).
State of matter (solid, liquid, or gas) applies at standard temperature and pressure conditions (STP).
Occurrence, as indicated by a footnote adjacent to the element's name, distinguishes naturally occurring elements, categorized as either primordial or transient (from decay), and additional synthetic elements that have been produced technologically, but are not known to occur naturally.
Color specifies an element's properties using the broad categories commonly presented in periodic tables: Actinide, alkali metal, alkaline earth metal, lanthanide, post-transition metal, metalloid, noble gas, polyatomic or diatomic nonmetal, and transition metal.

List of chemical elements
1	H	Hydrogen	composed of the Greek elements hydro- and -genmeaning 'water-forming'	1	1	1.008	0.00008988	14.01	20.28	14.304	2.20	1400
2	He	Helium	the Greek helios, 'sun'	18	1	4.002602(2)	0.0001785	—	4.22	5.193	–	0.008
3	Li	Lithium	the Greek lithos, 'stone'	1	2	6.94	0.534	453.69	1560	3.582	0.98	20
4	Be	Beryllium	beryl, a mineral	2	2	9.0121831(5)	1.85	1560	2742	1.825	1.57	2.8
5	B	Boron	borax, a mineral	13	2	10.81	2.34	2349	4200	1.026	2.04	10
6	C	Carbon	the Latin carbo, 'coal'	14	2	12.011	2.267	3800	4300	0.709	2.55	200
7	N	Nitrogen	the Greek nitron and '-gen' meaning 'niter-forming'	15	2	14.007	0.0012506	63.15	77.36	1.04	3.04	19
8	O	Oxygen	from the Greek oxy-, both 'sharp' and 'acid', and -gen, meaning 'acid-forming'	16	2	15.999	0.001429	54.36	90.20	0.918	3.44	461000
9	F	Fluorine	the Latin fluere, 'to flow'	17	2	18.998403163(6)	0.001696	53.53	85.03	0.824	3.98	585
10	Ne	Neon	the Greek neos, meaning 'new'	18	2	20.1797(6)	0.0008999	24.56	27.07	1.03	–	0.005
11	Na	Sodium	the English word soda (natrium in Latin)	1	3	22.98976928(2)	0.971	370.87	1156	1.228	0.93	23600
12	Mg	Magnesium	Magnesia, a district of Eastern Thessaly in Greece	2	3	24.305	1.738	923	1363	1.023	1.31	23300
13	Al	Aluminium	from alumina, a compound (originally alumium)	13	3	26.9815385(7)	2.698	933.47	2792	0.897	1.61	82300
14	Si	Silicon	from the Latin silex, 'flint' (originally silicium)	14	3	28.085	2.3296	1687	3538	0.705	1.9	282000
15	P	Phosphorus	the Greek phoosphoros, 'carrying light'	15	3	30.973761998(5)	1.82	317.30	550	0.769	2.19	1050
16	S	Sulfur	the Latin sulphur, 'fire and brimstone'	16	3	32.06	2.067	388.36	717.87	0.71	2.58	350
17	Cl	Chlorine	the Greek chloros, 'greenish yellow'	17	3	35.45	0.003214	171.6	239.11	0.479	3.16	145
18	Ar	Argon	the Greek argos, 'idle'	18	3	39.948(1)	0.0017837	83.80	87.30	0.52	–	3.5
19	K	Potassium	New Latin potassa, 'potash' (kalium in Latin)	1	4	39.0983(1)	0.862	336.53	1032	0.757	0.82	20900
20	Ca	Calcium	the Latin calx, 'lime'	2	4	40.078(4)	1.54	1115	1757	0.647	1	41500
21	Sc	Scandium	Scandia, the Latin name for Scandinavia	3	4	44.955908(5)	2.989	1814	3109	0.568	1.36	22
22	Ti	Titanium	Titans, the sons of the Earth goddess of Greek mythology	4	4	47.867(1)	4.54	1941	3560	0.523	1.54	5650
23	V	Vanadium	Vanadis, an Old Norse name for the Scandinavian goddess Freyja	5	4	50.9415(1)	6.11	2183	3680	0.489	1.63	120
24	Cr	Chromium	the Greek chroma, 'color'	6	4	51.9961(6)	7.15	2180	2944	0.449	1.66	102
25	Mn	Manganese	corrupted from magnesia negra, see Magnesium	7	4	54.938044(3)	7.44	1519	2334	0.479	1.55	950
26	Fe	Iron	English word (ferrum in Latin)	8	4	55.845(2)	7.874	1811	3134	0.449	1.83	56300
27	Co	Cobalt	the German word Kobold, 'goblin'	9	4	58.933194(4)	8.86	1768	3200	0.421	1.88	25
28	Ni	Nickel	from a mischievous sprite of German miner mythology, Nickel	10	4	58.6934(4)	8.912	1728	3186	0.444	1.91	84
29	Cu	Copper	English word (Latin cuprum)	11	4	63.546(3)	8.96	1357.77	2835	0.385	1.9	60
30	Zn	Zinc	German word Zinke (prong, tooth)	12	4	65.38(2)	7.134	692.88	1180	0.388	1.65	70
31	Ga	Gallium	Gallia, the Latin name for France	13	4	69.723(1)	5.907	302.9146	2673	0.371	1.81	19
32	Ge	Germanium	Germania, the Latin name for Germany	14	4	72.630(8)	5.323	1211.40	3106	0.32	2.01	1.5
33	As	Arsenic	English word (Latin arsenicum)	15	4	74.921595(6)	5.776	1090	887	0.329	2.18	1.8
34	Se	Selenium	the Greek selene, 'moon'	16	4	78.971(8)	4.809	453	958	0.321	2.55	0.05
35	Br	Bromine	the Greek bromos, 'stench'	17	4	79.904	3.122	265.8	332.0	0.474	2.96	2.4
36	Kr	Krypton	the Greek kryptos, 'hidden'	18	4	83.798(2)	0.003733	115.79	119.93	0.248	3	1×10
37	Rb	Rubidium	the Latin rubidus, 'deep red'	1	5	85.4678(3)	1.532	312.46	961	0.363	0.82	90
38	Sr	Strontium	Strontian, a small town in Scotland	2	5	87.62(1)	2.64	1050	1655	0.301	0.95	370
39	Y	Yttrium	Ytterby, Sweden	3	5	88.90584(2)	4.469	1799	3609	0.298	1.22	33
40	Zr	Zirconium	Persian Zargun, 'gold-colored'; German Zirkoon, 'jargoon'	4	5	91.224(2)	6.506	2128	4682	0.278	1.33	165
41	Nb	Niobium	Niobe, daughter of king Tantalus from Greek mythology	5	5	92.90637(2)	8.57	2750	5017	0.265	1.6	20
42	Mo	Molybdenum	the Greek molybdos meaning 'lead'	6	5	95.95(1)	10.22	2896	4912	0.251	2.16	1.2
43	Tc	Technetium	the Greek tekhnètos meaning 'artificial'	7	5	[98]	11.5	2430	4538	–	1.9	~ 3×10
44	Ru	Ruthenium	Ruthenia, the New Latin name for Russia	8	5	101.07(2)	12.37	2607	4423	0.238	2.2	0.001
45	Rh	Rhodium	the Greek rhodos, meaning 'rose coloured'	9	5	102.90550(2)	12.41	2237	3968	0.243	2.28	0.001
46	Pd	Palladium	the then recently discovered asteroid Pallas, considered a planet at the time	10	5	106.42(1)	12.02	1828.05	3236	0.244	2.2	0.015
47	Ag	Silver	English word (argentum in Latin)	11	5	107.8682(2)	10.501	1234.93	2435	0.235	1.93	0.075
48	Cd	Cadmium	the New Latin cadmia, from King Kadmos	12	5	112.414(4)	8.69	594.22	1040	0.232	1.69	0.159
49	In	Indium	indigo	13	5	114.818(1)	7.31	429.75	2345	0.233	1.78	0.25
50	Sn	Tin	English word (stannum in Latin)	14	5	118.710(7)	7.287	505.08	2875	0.228	1.96	2.3
51	Sb	Antimony	uncertain: perhaps from the Greek anti, 'against', and monos, 'alone', or the Old French antimoine, 'Monk's bane' (stibium in Latin)	15	5	121.760(1)	6.685	903.78	1860	0.207	2.05	0.2
52	Te	Tellurium	Latin tellus, 'earth'	16	5	127.60(3)	6.232	722.66	1261	0.202	2.1	0.001
53	I	Iodine	French iode (after the Greek ioeides, 'violet')	17	5	126.90447(3)	4.93	386.85	457.4	0.214	2.66	0.45
54	Xe	Xenon	the Greek xenos, 'strange'	18	5	131.293(6)	0.005887	161.4	165.03	0.158	2.6	3×10
55	Cs	Caesium	the Latin caesius, 'sky blue'	1	6	132.90545196(6)	1.873	301.59	944	0.242	0.79	3
56	Ba	Barium	the Greek barys, 'heavy'	2	6	137.327(7)	3.594	1000	2170	0.204	0.89	425
57	La	Lanthanum	the Greek lanthanein, 'to lie hidden'	3	6	138.90547(7)	6.145	1193	3737	0.195	1.1	39
58	Ce	Cerium	the then recently discovered asteroid Ceres, considered a planet at the time		6	140.116(1)	6.77	1068	3716	0.192	1.12	66.5
59	Pr	Praseodymium	the Greek praseios didymos meaning 'green twin'		6	140.90766(2)	6.773	1208	3793	0.193	1.13	9.2
60	Nd	Neodymium	the Greek neos didymos meaning 'new twin'		6	144.242(3)	7.007	1297	3347	0.19	1.14	41.5
61	Pm	Promethium	Prometheus of Greek mythology who stole fire from the Gods and gave it to humans		6	[145]	7.26	1315	3273	–	1.13	2×10
62	Sm	Samarium	Samarskite, the name of the mineral from which it was first isolated		6	150.36(2)	7.52	1345	2067	0.197	1.17	7.05
63	Eu	Europium	Europe		6	151.964(1)	5.243	1099	1802	0.182	1.2	2
64	Gd	Gadolinium	Johan Gadolin, chemist, physicist and mineralogist		6	157.25(3)	7.895	1585	3546	0.236	1.2	6.2
65	Tb	Terbium	Ytterby, Sweden		6	158.92535(2)	8.229	1629	3503	0.182	1.2	1.2
66	Dy	Dysprosium	the Greek dysprositos, 'hard to get'		6	162.500(1)	8.55	1680	2840	0.17	1.22	5.2
67	Ho	Holmium	Holmia, the New Latin name for Stockholm		6	164.93033(2)	8.795	1734	2993	0.165	1.23	1.3
68	Er	Erbium	Ytterby, Sweden		6	167.259(3)	9.066	1802	3141	0.168	1.24	3.5
69	Tm	Thulium	Thule, the ancient name for Scandinavia		6	168.93422(2)	9.321	1818	2223	0.16	1.25	0.52
70	Yb	Ytterbium	Ytterby, Sweden		6	173.045(10)	6.965	1097	1469	0.155	1.1	3.2
71	Lu	Lutetium	Lutetia, the Latin name for Paris		6	174.9668(1)	9.84	1925	3675	0.154	1.27	0.8
72	Hf	Hafnium	Hafnia, the New Latin name for Copenhagen	4	6	178.49(2)	13.31	2506	4876	0.144	1.3	3
73	Ta	Tantalum	King Tantalus, father of Niobe from Greek mythology	5	6	180.94788(2)	16.654	3290	5731	0.14	1.5	2
74	W	Tungsten	the Swedish tung sten, 'heavy stone' (W is wolfram, the old name of the tungsten mineral wolframite)	6	6	183.84(1)	19.25	3695	5828	0.132	2.36	1.3
75	Re	Rhenium	Rhenus, the Latin name for the river Rhine	7	6	186.207(1)	21.02	3459	5869	0.137	1.9	7×10
76	Os	Osmium	the Greek osmè, meaning 'smell'	8	6	190.23(3)	22.61	3306	5285	0.13	2.2	0.002
77	Ir	Iridium	Iris, the Greek goddess of the rainbow	9	6	192.217(3)	22.56	2719	4701	0.131	2.2	0.001
78	Pt	Platinum	the Spanish platina, meaning 'little silver'	10	6	195.084(9)	21.46	2041.4	4098	0.133	2.28	0.005
79	Au	Gold	English word (aurum in Latin)	11	6	196.966569(5)	19.282	1337.33	3129	0.129	2.54	0.004
80	Hg	Mercury	the New Latin name mercurius, named after the Roman god (Hg from former name hydrargyrum, from Greek hydr-, 'water', and argyros, 'silver')	12	6	200.592(3)	13.5336	234.43	629.88	0.14	2	0.085
81	Tl	Thallium	the Greek thallos, 'green twig'	13	6	204.38	11.85	577	1746	0.129	1.62	0.85
82	Pb	Lead	English word (plumbum in Latin)	14	6	207.2(1)	11.342	600.61	2022	0.129	1.87	14
83	Bi	Bismuth	Uncertain, possibly Arabic or German	15	6	208.98040(1)	9.807	544.7	1837	0.122	2.02	0.009
84	Po	Polonium	Named after the home country of Marie Curie(Polonia, Latin for Poland), who is also the discoverer of Radium	16	6	[209]	9.32	527	1235	–	2.0	2×10
85	At	Astatine	the Greek astatos, 'unstable'	17	6	[210]	7	575	610	–	2.2	3×10
86	Rn	Radon	From radium, as it was first detected as an emission from radium during radioactive decay	18	6	[222]	0.00973	202	211.3	0.094	2.2	4×10
87	Fr	Francium	Francia, the New Latin name for France	1	7	[223]	1.87	300	950	–	0.7	~ 1×10
88	Ra	Radium	the Latin radius, 'ray'	2	7	[226]	5.5	973	2010	0.094	0.9	9×10
89	Ac	Actinium	the Greek aktis, 'ray'	3	7	[227]	10.07	1323	3471	0.12	1.1	5.5×10
90	Th	Thorium	Thor, the Scandinavian god of thunder		7	232.0377(4)	11.72	2115	5061	0.113	1.3	9.6
91	Pa	Protactinium	the Greek protos, 'first', and actinium, which is produced through the radioactive decay of protactinium		7	231.03588(2)	15.37	1841	4300	–	1.5	1.4×10
92	U	Uranium	Uranus, the seventh planet in the Solar System		7	238.02891(3)	18.95	1405.3	4404	0.116	1.38	2.7
93	Np	Neptunium	Neptune, the eighth planet in the Solar System		7	[237]	20.45	917	4273	–	1.36	≤ 3×10
94	Pu	Plutonium	Pluto, a dwarf planet in the Solar System (then considered the ninth planet)		7	[244]	19.84	912.5	3501	–	1.28	≤ 3×10
95	Am	Americium	The Americas, as the element was first synthesized on the continent, by analogy with europium		7	[243]	13.69	1449	2880	–	1.13	0
96	Cm	Curium	Pierre Curie, a physicist, and Marie Curie, a physicist and chemist, named after great scientists by analogy with gadolinium		7	[247]	13.51	1613	3383	–	1.28	0
97	Bk	Berkelium	Berkeley, California, where the element was first synthesized, by analogy with terbium		7	[247]	14.79	1259	2900	–	1.3	0
98	Cf	Californium	California, where the element was first synthesized		7	[251]	15.1	1173	(1743)	–	1.3	0
99	Es	Einsteinium	Albert Einstein, physicist		7	[252]	8.84	1133	(1269)	–	1.3	0
100	Fm	Fermium	Enrico Fermi, physicist		7	[257]	(9.7)	(1125)	–	–	1.3	0
101	Md	Mendelevium	Dmitri Mendeleev, chemist and inventor		7	[258]	(10.3)	(1100)	–	–	1.3	0
102	No	Nobelium	Alfred Nobel, chemist, engineer, inventor of dynamite		7	[259]	(9.9)	(1100)	–	–	1.3	0
103	Lr	Lawrencium	Ernest O. Lawrence, physicist		7	[266]	(15.6)	(1900)	–	–	1.3	0
104	Rf	Rutherfordium	Ernest Rutherford, chemist and physicist	4	7	[267]	(23.2)	(2400)	(5800)	–	–	0
105	Db	Dubnium	Dubna, Russia	5	7	[268]	(29.3)	–	–	–	–	0
106	Sg	Seaborgium	Glenn T. Seaborg, scientist	6	7	[269]	(35.0)	–	–	–	–	0
107	Bh	Bohrium	Niels Bohr, physicist	7	7	[270]	(37.1)	–	–	–	–	0
108	Hs	Hassium	Hesse, Germany, where the element was first synthesized	8	7	[277]	(40.7)	–	–	–	–	0
109	Mt	Meitnerium	Lise Meitner, physicist	9	7	[278]	(37.4)	–	–	–	–	0
110	Ds	Darmstadtium	Darmstadt, Germany, where the element was first synthesized	10	7	[281]	(34.8)	–	–	–	–	0
111	Rg	Roentgenium	Wilhelm Conrad Röntgen, physicist	11	7	[282]	(28.7)	–	–	–	–	0
112	Cn	Copernicium	Nicolaus Copernicus, astronomer	12	7	[285]	(23.7)	–	~357	–	–	0
113	Nh	Nihonium	the Japanese name for Japan, Nihon, where the element was first synthesized	13	7	[286]	(16)	(700)	(1400)	–	–	0
114	Fl	Flerovium	Flerov Laboratory of Nuclear Reactions, part of JINRwhere the element was synthesized; itself named for Georgy Flyorov, physicist	14	7	[289]	(14)	–	~210	–	–	0
115	Mc	Moscovium	Moscow Oblast, Russia, where the element was first synthesized	15	7	[290]	(13.5)	(700)	(1400)	–	–	0
116	Lv	Livermorium	Lawrence Livermore National Laboratory (in Livermore, California) which collaborated with JINRon its synthesis	16	7	[293]	(12.9)	(709)	(1085)	–	–	0
117	Ts	Tennessine	Tennessee, United States	17	7	[294]	(7.2)	(723)	(883)	–	–	0
118	Og	Oganesson	Yuri Oganessian, physicist	18	7	[294]	(5.0)	–	(350)	–	–	0
Notes Z is the standard symbol for atomic number; C is the standard symbol for heat capacity; and χ is the standard symbol for electronegativity on the Pauling scale. Unless otherwise indicated, elements are primordial – they occur naturally, and not through decay. The isotopic composition of this element varies in some geological specimens, and the variation may exceed the uncertainty stated in the table. The isotopic composition of the element can vary in commercial materials, which can cause the atomic weight to deviate significantly from the given value. The isotopic composition varies in terrestrial material such that a more precise atomic weight can not be given. The value listed is the conventional atomic-weight value suitable for trade and commerce. The actual value may differ depending on the isotopic composition of the sample. Since 2009, IUPAC provides the standard atomic-weight values for these elements using the interval notation. The corresponding standard atomic weights are: Hydrogen: [1.00784, 1.00811] Lithium: [6.938, 6.997] Boron: [10.806, 10.821] Carbon: [12.0096, 12.0116] Nitrogen: [14.00643, 14.00728] Oxygen: [15.99903, 15.99977] Magnesium: [24.304, 24.307] Silicon: [28.084, 28.086] Sulfur: [32.059, 32.076] Chlorine: [35.446, 35.457] Bromine: [79.901, 79.907] Thallium: [204.382, 204.385] Helium does not solidify at a pressure of one atmosphere. Helium can only solidify at pressures above 25 atmospheres, which corresponds to a melting point of absolute zero. The atomic weight of commercial lithium can vary between 6.939 and 6.996—analysis of the specific material is necessary to find a more accurate value. This element sublimes at one atmosphere of pressure. The element does not have any stable nuclides, and a value in brackets, e.g. [209], indicates the mass number of the longest-lived isotope of the element. However, four such elements, bismuth, thorium, protactinium, and uranium, have characteristic terrestrial isotopic compositions, and thus their standard atomic weights are given. This element is transient – it occurs only through decay. This element is synthetic – the transuranic elements 95 and above do not occur naturally, but they can all be produced artificially. The value has not been precisely measured, usually because of the element's short half-life; the value given in parentheses is a prediction. With error bars: 357+112 −108 K. This predicted value is for liquid oganesson, not gaseous oganesson.