<P> Lithium is heated to a temperature of about 550 ° C in a vacuum . Lithium atoms evaporate from the liquid surface and are collected on a cold surface positioned a few centimetres above the liquid surface . Since lithium - 6 atoms have a greater mean free path, they are collected preferentially . </P> <P> The theoretical separation efficiency is about 8.0 percent . A multistage process may be used to obtain higher degrees of separation . </P> <P> Lithium - 4 contains three protons and one neutron . It is the shortest - lived known isotope of lithium, with a half - life of about 91 yoctoseconds, 6977909999999999999 ♠ 9.1 × 10 seconds and decays by proton emission to helium - 3 . Lithium - 4 can be formed as an intermediate in some nuclear fusion reactions . </P> <P> Lithium - 6 is valuable as the source material for the production of tritium (hydrogen - 3) and as an absorber of neutrons in nuclear fusion reactions . Natural lithium contains about 7.5 percent lithium - 6, with the rest being lithium - 7 . Large amounts of lithium - 6 have been separated out for placing into hydrogen bombs . The separation of lithium - 6 has by now ceased in the large thermonuclear powers, but stockpiles of it remain in these countries . Lithium - 6 is one of only three stable isotopes with a spin of 1 and has the smallest nonzero nuclear electric quadrupole moment of any stable nucleus . </P>

How many protons are in a lithium nucleus