What is atomic mass? This is basically the weighted average of the isotopes for a particular element. In chemistry, Isotopes are the atoms having similar atomic number (no of protons) but number of neutrons are generally different. With the total count of protons, you can check the identity of an atom, so isotopes generally have same atomic numbers, so they are atoms of same element.
At the same time, the total number of neutrons would be equal to the masses of isotopes. The atomic mass for some element is measured on AMU (Atomic Mass Units) where one AMU is generally equivalent to the mass of a single proton and neutron. Isotopes don’t exist in similar ratio always, if they occur in higher percentage then it could affect the atomic masses largely.
There are three methods to know the atomic mass, depending on one’s circumstances.
How to calculate the atomic masses?
On the other hand, if isotopes occur in lower percentage then it would not affect the atomic masses much. If you wanted to calculate the atomic mass then multiply the mass of isotopes together and add the individual masses together.
In other words, when proton, electrons, and neutrons of one atom are added together then this is named as average mass of the atom. As compared to neutrons and the protons, electrons are so limited in count that it has only a small affect on the calculation. So, the total of neutrons and protons is the atomic mass generally.