The total number of protons and neutrons in an atom is called its mass number.
The atomic number is shown below the chemical symbol, and the mass number is shown above. In this example the atomic number is 17 and the mass number is 35. This means that each of these atoms has:
Isotopes are atoms of an element with the normal number of protons (and electrons), but different numbers of neutrons. Isotopes have the same atomic number, but different mass numbers.
Most hydrogen atoms consist of just one proton and one electron, but some also have one or two neutrons.