also read module 01
Every atom has its own mass number: the sum of the nuclear particles.

The Cl atom has 17 protons and can have 18 neutrons, or: the mass number of this Chlorine atom is 35.

Any element can have various isotopes, particles with equal atom number, but different mass number. The average atom mass must be based upon the proportion between those isotopes.
The values of atom masses can be found in tables, like table V, column 4
The atom mass is based upon the mass number of de different isotopes of the element.

Chlorine, for example, has apart from atom 35, another isotope with 20 neutrons, so with mass number 37.

The average is not 36, because in nature there is three times more 35Cl than 37Cl.

Finally you can calculate that the average atom mass of Chlorine must be about 35.5

It is calculated that 6.1023 nuclear particles (toghether weighting 6.1023 u), have a total mass of one gramme.
1 gramme contains: 600. nuclear particles (nucleons, meaning the protons and the neutrons toghether)

The average mass of one nuclear particle equals 1,7.10-24 gramme.