Chemistry problem help?

I'm doing some practice problems, and it says to figure out the # of protons, neutrons, and electrons in Lithium, with an atomic number of 3 and an atomic mass of 7. So I said the protons and electrons were 3 and the neutrons were 4. However, the answers in the back of the book say 12 protons and electrons and 13 neutrons... I don't know how they got that Help?