Isotope

Definition ;
Isotopes are atoms containing the same number of protons and electrons but a different number of neutrons. Some common isotopes are chlorine 35, chlorine 37 and Hydrogen:-1,2 and 3. 

Chlorines average mass is calculated;
Cl 35 has 75% and cl 37 has 25% of it's mass.

Chlorines Ar of 35.5, which is calculated by (75 x 35) + (25 x37)/100 = 35.3