Lithium has an atomic number of 3 and an average atomic mass of 6.941 amu. All lithium atoms have 3 protons in the nucleus. A neutral atom of lithium always has 3 electrons to balance the charge of the 3 protons. The average atomic mass of a lithium atom is 6.941 amu. This indicates that some naturally occurring lithium atoms have 3 neutrons, to make a total atomic mass of 6 amu. Others have 4 neutrons, to make a total atomic mass of 7 amu. These two isotopes are designated lithium-6 and lithium-7. Since there are so many more lithium-7 atoms, the average of all of the atoms is very close to 7 amu.