<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> <P> Ten materials familiar to various prehistoric cultures are now known to be chemical elements: Carbon, copper, gold, iron, lead, mercury, silver, sulfur, tin, and zinc . Three additional materials now accepted as elements, arsenic, antimony, and bismuth, were recognized as distinct substances prior to 1500 AD . Phosphorus, cobalt, and platinum were isolated before 1750 . </P> <P> Most of the remaining naturally occurring chemical elements were identified and characterized by 1900, including: </P>

An element or compound which occurs naturally in the earth is a