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In chemistry, the chemical elements labeled as synthetic are too unstable to be found naturally on Earth. These synthetic elements possess half-lives so short, relative to the age of the Earth, that any atoms of these elements that may have existed when the Earth formed have long since decayed away. Because of this, atoms of synthetic elements are only present on Earth as the product of experiments involving nuclear reactors or particle accelerators via nuclear fusion or neutron absorption. Uranium and thorium have no stable isotopes, but are found naturally in the Earth's crust and atmosphere, so neither of these two elements are called synthetic. Unstable elements such as polonium, radium, and radon—which are formed through the decay of uranium and thorium—can also be found in nature despite having very short half-lives.

The first element discovered through synthesis was technetium. 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 found naturally in red giant stars.

Atomic mass for natural elements is based on weighted average abundance of natural isotopes occurring in the Earth's crust and atmosphere. For synthetic elements the isotope formed depends on the means of synthesis, so the concept of natural isotope abundance has no meaning. Therefore for synthetic elements the total nucleus (protons plus neutrons) count of the most stable isotope (ie, the isotope with the longest half-life) is listed in brackets as the atomic mass.

Quasi-synthetic^{[clarification needed]} elements include:

• Technetium, atomic number 43
• Promethium, atomic number 61
• Astatine, atomic number 85
• Francium, atomic number 87
• Neptunium, atomic number 93
• Plutonium, atomic number 94

(All elements with atomic numbers 1 through 94 are naturally occurring at least in trace quantities)

• Transuranium elements
  • Americium symbol Am, atomic number 95
  • Curium symbol Cm, atomic number 96
  • Berkelium symbol Bk, atomic number 97
  • Californium symbol Cf, atomic number 98
  • Einsteinium symbol Es, atomic number 99
  • Fermium symbol Fm, atomic number 100
  • Mendelevium symbol Md, atomic number 101
  • Nobelium symbol No, atomic number 102
  • Lawrencium symbol Lr, atomic number 103

• Transactinide elements
  • Rutherfordium symbol Rf, atomic number 104
  • Dubnium symbol Db, atomic number 105
  • Seaborgium symbol Sg, atomic number 106
  • Bohrium symbol Bh, atomic number 107
  • Hassium symbol Hs, atomic number 108
  • Meitnerium symbol Mt, atomic number 109
  • Darmstadtium symbol Ds, atomic number 110
  • Roentgenium symbol Rg, atomic number 111

Provisional names for recently observed synthetic elements:

• Ununbium symbol Uub, atomic number 112
• Ununtrium symbol Uut, atomic number 113
• Ununquadium symbol Uuq, atomic number 114
• Ununpentium symbol Uup, atomic number 115
• Ununhexium symbol Uuh, atomic number 116
• Ununoctium symbol Uuo, atomic number 118