While we are looking at the Periodic Table, you may also notice that the atomic weight doesn't doesn't match the number of protons. This is because of the presence of neutrons, which have no charge associated with them.
Compare hydrogen and helium. Hydrogen's atomic number is 1, meaning it has one proton. It also has a mass number of 1 amu. Recall, protons only have a mass of 1 amu, so hydrogen has only one proton and no neutrons. Helium, on the other hand, has an atomic number of 2 and mass number of 4. So what causes this increase? Helium's atomic number tells us it has 2 protons. If you take the mass number (4) and subtract the number of protons (2), you will find that helium has 2 neutrons..