Isotopes
Atoms of an element with the same number of protons and electrons but different numbers of neutrons. are atoms with the same atomic number but different mass number.
These two carbon atoms are isotopes. Each has the same number of protons but a different number of neutrons.
Relative atomic mass
Elements are made up of isotopes so the mass given in the data booklet is called the relative atomic mass (RAM).
The relative atomic mass of each element shown in the data book is calculated from the masses of all the isotopes of an element taking into account the percentage proportion of each.
Example
There are two isotopes of chlorine:
Since the RAM of chlorine is 35.5, chlorine-35 must be more abundant because its mass number is closer to the RAM.