The atomic number is the number of protons of an atom and, with other information, can be used to determine the number of electrons and the number of neutrons in an atom.

Oxygen for example has the atomic number 8. Therefore, an oxygen atom has 8 protons in its nucleus. A charge-neutral oxygen atom would also have 8 electrons in its energy shells.

The is the number of protons plus the number of neutrons in an element, thus the atomic number can help us determine the atomic mass or the number of neutrons in an element.

For example, in the image below, if we know the mass number (atomic mass )is 74 and we know the atomic number/number of protons in the element, we can solve for the number of neutrons. 74-32=neutrons