<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> <P> The discoveries of mendelevium, nobelium, lawrencium followed . During the height of the Cold War, the Soviet Union and United States independently discovered rutherfordium and dubnium . The naming and credit for discovery of those elements remained unresolved for many years but eventually shared credit was recognized by IUPAC / IUPAP in 1992 . In 1997, IUPAC decided to give dubnium its current name honoring the city of Dubna where the Russian team made their discoveries since American - chosen names had already been used for many existing synthetic elements, while the name rutherfordium (chosen by the American team) was accepted for element 104 . </P>

Who was the first chemist to lead a team that made elements that did not occur naturally