<P> In 1661, Robert Boyle proposed his theory of corpuscularism which favoured the analysis of matter as constituted by irreducible units of matter (atoms) and, choosing to side with neither Aristotle's view of the four elements nor Paracelsus' view of three fundamental elements, left open the question of the number of elements . The first modern list of chemical elements was given in Antoine Lavoisier's 1789 Elements of Chemistry, which contained thirty - three elements, including light and caloric . By 1818, Jöns Jakob Berzelius had determined atomic weights for forty - five of the forty - nine then - accepted elements . Dmitri Mendeleev had sixty - six elements in his periodic table of 1869 . </P> <P> From Boyle until the early 20th century, an element was defined as a pure substance that could not be decomposed into any simpler substance . Put another way, a chemical element cannot be transformed into other chemical elements by chemical processes . Elements during this time were generally distinguished by their atomic weights, a property measurable with fair accuracy by available analytical techniques . </P> <P> 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 10 seconds it takes the nucleus to form an electronic cloud . </P> <P> By 1914, seventy - two elements were known, all naturally occurring . 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 in a periodic manner . Most recently, the synthesis of element 118 was reported in October 2006, and the synthesis of element 117 was reported in April 2010 . </P>

Who gave the first working definition of an element