The mass # is used to calculate the number of__in 1 atom of an element. In order to calculate the # of neutrons you must subtract the__from...

The atomic mass of an atom is the sum of the number of
protons and neutrons that it has in its nucleus. The mass of protons and neutron varies
slightly as is generally taken as 1, and the mass of electrons is taken as 0 as it is
very small compared to nucleons.

So to find the number of
neutrons you have to subtract the atomic number from the atomic
mass.

The mass # is used to calculate the number of
nucleons in 1 atom of an element. In order to calculate the # of neutrons you must
subtract the atomic number from the atomic mass.