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]
History of the periodic table Alexandre-Emile Béguyer de Chancourtois, a French geologist, was the first person to notice the periodicity of the elements — similar elements occurring at regular intervals when they are ordered by their atomic weights. In 1862 he devised an early form of periodic table, which he named Vis tellurique (the 'telluric helix'), after the element tellurium, which fell near the center of his diagram.[10][11] With the elements arranged in a spiral on a cylinder by order of increasing atomic weight, de Chancourtois saw that elements with similar properties lined up vertically. His 1863 publication included a chart (which contained ions and compounds,[12] in addition to elements), but his original paper in the Comptes Rendus de l'Académie des Sciences used geological rather than chemical terms and did not include a diagram. As a result, de Chancourtois' ideas received little attention until after the work of Dmitri Mendeleev had been publicised.[13]