A chemical element, or element, is a type of
atom that is defined by its
atomic number; that is, by the number of
protons in its
nucleus. The term is also used to refer to a pure chemical
substance composed of atoms with the same number of protons.Common examples of elements are
hydrogen,
nitrogen, and
carbon. In total, 117 elements have been observed as of 2007, of which 94 occur naturally on Earth. Elements with atomic numbers greater than 82 (i.e,.
bismuth and those above), are inherently unstable and undergo radioactive decay. In addition, elements 43 and 61 (technetium and
promethium) have no stable
isotopes, and also decay. However, even the elements up to atomic number 94 with no stable nuclei are nevertheless found in nature, as a result of the natural decay processes of
uranium and
thorium.
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A fundamental substance comprising one kind of atom; the simplest form of matter.
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