The atomic number (also called the proton number) is the
number of protons in an atom.
The mass number (also called the nucleon number) is the
total number of protons and neutrons in an atom.
Proton: charge = +1 mass = 1 Neutron: charge = 0 mass = 1
Electron: charge: -1 mass: 0.0005 (almost zero)
Most elements exist in a number of different forms or
varieties called isotopes. Isotopes have the same number of
protons but different numbers of neutrons, which gives them
a different total mass.