there are 5 isotopes of lithium having respectively 2, 3, 4, 5 and 6 neutrons in the nucleus. the most common isotope in nature is 3li7 which makes up 92.58% of the total. the second isotope which is widely available is 3li6 which makes up 7.42% of the total. the other 3 isotopes exist in very small quantities. the atomic mass of lithium is 6.939.