Isotopes 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.
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.
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.