who gave the first working definition of an element
Chemical element The 1913 discovery by English physicist Henry Moseley that the nuclear charge is the physical basis for an atom's atomic number, further refined when the nature of protons and neutrons became appreciated, eventually led to the current definition of an element based on atomic number (number of protons per atomic nucleus). 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 allotropes within the same element. Currently, IUPAC defines an element to exist if it has isotopes with a lifetime longer than the 10âˆ’14 seconds it takes the nucleus to form an electronic cloud.[29]
Dmitri Mendeleev Dmitri Ivanovich Mendeleev[2] (English: /ˌmɛndəlˈeɪəf/ MEN-dəl-AY-əf;[3] Russian: Дмитрий Иванович Менделеев[note 1], tr. Dmítriy Ivánovich Mendeléyev, IPA: [ˈdmʲitrʲɪj ɪˈvanəvʲɪtɕ mʲɪndʲɪˈlʲejɪf] ( listen); 8 February 1834 – 2 February 1907 O.S. 27 January 1834 – 20 January 1907) was a Russian chemist and inventor. He formulated the Periodic Law, created a farsighted version of the periodic table of elements, and used it to correct the properties of some already discovered elements and also to predict the properties of eight elements yet to be discovered.