Editing Chemical element (section)

=== Isotopes ===
{{Main|Isotope|Stable isotope ratio|List of nuclides}}

[[Isotope]]s are atoms of the same element (that is, with the same number of [[proton]]s in their nucleus), but having ''different'' numbers of [[neutron]]s. Thus, for example, there are three main isotopes of carbon. All carbon atoms have 6 protons, but they can have either 6, 7, or 8 neutrons. Since the mass numbers of these are 12, 13 and 14 respectively, said three isotopes are known as [[carbon-12]], [[carbon-13]], and [[carbon-14]] ({{sup|12}}C, {{sup|13}}C, and {{sup|14}}C). Natural carbon is a [[mixture]] of {{sup|12}}C (about 98.9%), {{sup|13}}C (about 1.1%) and about 1 atom per trillion of {{sup|14}}C.

Most (54 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 elements have radioactive isotopes (radioisotopes); most of these radioisotopes do not occur naturally. Radioisotopes typically decay into other elements via [[alpha decay]], [[beta decay]], or [[inverse beta decay]]; some isotopes of the heaviest elements also undergo [[spontaneous fission]]. Isotopes that are not radioactive, are termed "stable" isotopes. All known stable isotopes occur naturally (see [[primordial nuclide]]). The many radioisotopes that are not found in nature have been characterized after being artificially produced. Certain elements have no stable isotopes and are composed ''only'' of radioisotopes: specifically the elements without any stable isotopes are technetium (atomic number 43), promethium (atomic number 61), and all observed elements with atomic number greater than 82.

Of the 80 elements with at least one stable isotope, 26 have only one 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 for a single element is 10 (for [[tin]], element 50).