The major portion of an atomic mass consist of

The major portion of an atomic mass consists of neutrons and protons.

neutrons and protons

Neutrons and Protons
However, when you have a isotope, the number of neutrons will change, therefore, making the atomic mass different. 
Atomic Mass= Number of neutrons plus number of protons.
You can find the number of protons by looking at the atomic number.
Then, subtract the number of protons by the mass number, therefore, giving you the number of neutrons.
It is kind of like a fact family. 

4hours and 17 mins i think