A chemical element (sometimes called simply element) is a material that consists of atoms with specific numbers of protons in their nuclei. This number is known as the atomic number of the element. For example, all atoms with 6 protons in their nuclei are atoms of the chemical element carbon, and all atoms with 92 protons in their nuclei are atoms of the element uranium. A number of elements are radioactive and in undergoing radioactive decay transmute into a different element.
There are as of 2004, 116 known elements, only 91 of which are naturally occurring. The balance are only found on Earth when man-made; the first such element was 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 element 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 did actually discover an element, and 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. 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.