Lithium (Li) is the element with atomic number 3, symbol Li. The most abundant isotope, mass number 7, is stable, and one more isotope, mass number 6 is also stable.
Lithium is thought to have been formed in the Big Bang, but there seems to be more than it would produce. One possible source is cosmic ray spallation in the interstellar medium, i.e., fission produced by cosmic rays. Another possibility is neutrino-induced processes in core collapse supernovae.
The Sun shows a lower abundance of lithium than expected from Big Bang nucleosynthesis, which was long a mystery, i.e., the solar lithium problem. It burns at a temperature less than hydrogen burning, and it is believed that young stars continue mixing their constituents sufficiently long that the lithium is burned in the star's interior early in the star's life.