How is atomic number and mass related to protons, neutrons, & electrons?

Atomic number . . .
-- the number of protons in an atom
-- the number of electrons in a neutral atom
-- neutrons are not involved

Atomic mass . . .
-- roughly but not exactly the total of (protons + neutrons) in an atom
-- electrons are not involved

The mass has escaped into a gas which you can not see so the law stays true.