<P> Synthetic elements are radioactive and decay rapidly into lighter elements--possessing half - lives so short, relative to the age of Earth (which formed approximately 4.6 billion years ago), that any atoms of these elements that may have existed when the Earth formed have long since decayed . Atoms of synthetic elements only occur on Earth as the product of atomic bombs or experiments that involve nuclear reactors or particle accelerators, via nuclear fusion or neutron absorption . </P> <P> Atomic mass for natural elements is based on weighted average abundance of natural isotopes that occur in Earth's crust and atmosphere . For synthetic elements, the isotope depends on the means of synthesis, so the concept of natural isotope abundance has no meaning . Therefore, for synthetic elements the total nucleon count (protons plus neutrons) of the most stable isotope, i.e. the isotope with the longest half - life--is listed in brackets as the atomic mass . </P> <P> The first element discovered through synthesis was technetium--its discovery being definitely confirmed in 1936 . This discovery filled a gap in the periodic table, and the fact that no stable isotopes of technetium exist explains its natural absence on Earth (and the gap). With the longest - lived isotope of technetium, Tc - 98, having a 4.2 - million - year half - life, no technetium remains from the formation of the Earth . Only minute traces of technetium occur naturally in the Earth's crust--as a spontaneous fission product of uranium - 238 or by neutron capture in molybdenum ores--but technetium is present naturally in red giant stars . </P> <P> The first discovered synthetic element was curium, synthesized in 1944 by Glenn T. Seaborg, Ralph A. James, and Albert Ghiorso by bombarding plutonium with alpha particles . The discoveries of americium, berkelium, and californium followed soon . Einsteinium and fermium were discovered by a team of scientists led by Albert Ghiorso in 1952 while studying the radioactive debris from the detonation of the first hydrogen bomb . The isotopes discovered were einsteinium - 253, with a half - life of 20.5 days, and fermium - 255, with a half - life of about 20 hours . </P>

What is the short form that chemists use to identify an element