Well first of all the atomic mass is the number of protons + the number of neutrons this is the mass of the nucleus and meaning that the protons and neutrons make up the nucleus which is where all the mass is located. So the number of protons will be equal to the number of electrons if it is a neutral atom. So the atomic number of Lithium is 3 and this is equal to the number of protons. So if you want to find the number of neutrons you have to subtract the number of protons which in this case is 3, so you're going to subtract 3 from 6 (6-3) and this is going to leave you with 3 neutrons. So in this case there are 3 Protons, 3 Neutrons, And 3 Electrons.
