Atoms and Isotopes
Because protons and neutrons have the same mass, their relative masses are 1.
An electron's mass relative to these is negligable, so the mass of an atom is considered to be made up of protons and neutrons.
The number of protons plus neutrons in an atom is called the mass number.
In an element, all the atoms have the same number of protons but the number of neutrons can change. Atoms in the same element with different number of neutrons are called isotopes.
The number of neutrons can be found by subtracting the atomic number from the mass number.