Chemical substance can be transmuted into another element through a nuclear reaction. This is so, because all of the atoms in a sample of an element have the same number of protons, though they may be different isotopes, with differing numbers of neutrons. As of 2012, there are 118 known elements, about 80 of which are stable – that is, they do not change by radioactive decay into other elements. Some elements can occur as more than a single chemical substance (allotropes). For instance, oxygen exists as both diatomic oxygen (O) and ozone (O). The majority of elements are classified as metals.
Chemical nomenclature that each substance has a single name, although a limited number of alternative names is acceptable in some cases. Preferably, the name also conveys some information about the structure or chemistry of a compound. The American Chemical Society's CAS numbers form an extreme example of names that do not perform this function: each CAS number refers to a single compound but none contain information about the structure. The form of nomenclature used depends on the audience to which it is addressed. As such, no single correct form exists, but rather there are different forms that are more or less appropriate