Editing Chemical element (section)

=== Atomic definitions ===
[[File:Henry Moseley.jpg|thumb|upright=1.0|[[Henry Moseley]]]]

The 1913 discovery by English physicist [[Henry Moseley]] that the nuclear charge is the physical basis for the atomic number, further refined when the nature of protons and [[neutron]]s became appreciated, eventually led to the current definition of an element based on atomic number (number of protons). The use of atomic numbers, rather than atomic weights, to distinguish elements has greater predictive value (since these numbers are integers) and also resolves some ambiguities in the chemistry-based view due to varying properties of isotopes and [[allotrope]]s within the same element. Currently, IUPAC defines an element to exist if it has isotopes with a lifetime longer than the 10{{sup|−14}} seconds it takes the nucleus to form an electronic cloud.<ref>[http://www.kernchemie.de/Transactinides/Transactinide-2/transactinide-2.html Transactinide-2] {{Webarchive|url=https://web.archive.org/web/20160303222840/http://www.kernchemie.de/Transactinides/Transactinide-2/transactinide-2.html |date=3 March 2016 }}. www.kernchemie.de</ref>

By 1914, eighty-seven elements were known, all naturally occurring (see [[Discovery of chemical elements]]). The remaining naturally occurring elements were discovered or isolated in subsequent decades, and various additional elements have also been produced synthetically, with much of that work pioneered by [[Glenn T. Seaborg]]. In 1955, element 101 was discovered and named [[mendelevium]] in honor of D. I. Mendeleev, the first to arrange the elements periodically.