A chemical element, often called simply element, is a substance that cannot be divided or changed into different substances by ordinary chemical methods. The smallest particle of such an element is an atom, which consists of electrons centered around a nucleus of protons and neutrons.
The atomic number of an element, Z, is equal to the number of protons in the atom of the element. For example, carbon, the element with atomic number 6, contains 6 protons in its nucleus. All atoms of an element have the same atomic number and contain the same number of protons. However, atoms of the same element may differ in the number of neutrons, and are known as isotopes of the element. The atomic mass of an element, A, is measured in atomic mass units (amu) and is roughly equal to the sum of the protons and neutrons in the nucleus of an atom of the element. A number of elements are radioactive and in undergoing radioactive decay transmute into a different element.
The lightest elements are hydrogen and helium, which were the first elements to appear in the Big Bang. All the heavier elements are made naturally and artificially through various methods of nucleosynthesis.
There are as of 2004, 116 known elements, only 91 of which occur naturally. The remaining 25 are man-made; the first such element being Technetium in 1937. All man-made elements are radioactive with short half-lives so that any that were present at the formation of Earth have long since decayed.
Lists of the elements by name, by symbol, and by atomic number are available. The most convenient presentation of the elements is in the periodic table, which groups elements with similar chemical properties together.
Atoms of the same element whose nuclei contain a different number of neutrons are said to be different isotopes of the element. A pure can exist as monatomic units or as diatomic or polyatomic units comprising the same kind of atoms. These are called allotropes, irrespective of the state.
The official names of the chemical elements are decided by the International Union of Pure and Applied Chemistry, which generally adopts the name chosen by the discoverer. This can lead to the controversial question of which research group actually discovered an element, a question which delayed the naming of elements with atomic number of 104 and higher for a considerable time. (See element naming controversy) Chemical elements are also given a unique chemical symbol, based on the name of the element, not necessarily in English. (For example, carbon has chemical symbol 'C', and sodium has chemical symbol 'Na' after the Latin natrium). Chemical symbols are understood internationally when element names might need to be translated. The first letter of a chemical symbol is always capitalized, as in the preceding examples. The full name of the element is not capitalized, even if it is derived from a proper noun (unless it would be capitalized by some other rule, for instance if it begins a sentence).
Elements can combine (react) to form pure compounds (such as water, salts, oxides and organic compounds). In many cases these compounds have essentially one fixed stoichiometry (composition) and their own structure and properties.
Some elements, particularly metallic elements, combine to form new structures with a more variable composition (such as metal alloys). In those cases it is better to speak of phases rather than compounds.
In general, a particular chemical can consist of a mixture of all of the above.