So # of protons is always going to just be the atomic number, which if you look on a periodic table, its the big one in the corner. So you can look that up easy for Li, Ca, and Se.
The numbers tell you the atomic mass over the atomic number.
Since the two particles that give an atom mass (neutrons and protons) both have a mass of about 1 amu, the atomic mass is essentially # of neutrons + number of protons.
So # of neutrons = atomic mass - number of protons.
The number of electrons is the same as the number of protons unless the atom is actually an ion.
If so, then if you had something like Li, it would have 3 electrons (same as # of protons). But if you had Li+, it would only have two. The plus shows that you lost one negative charge (one electron), hence why you have 2. <span />
