To find the number of neutrons we would subtract the number of protons from mass number of an atom.
Explanation:
An atom consist of electron, protons and neutrons. Protons and neutrons are present with in nucleus while the electrons are present out side the nucleus.
All these three subatomic particles construct an atom. A neutral atom have equal number of proton and electron. In other words we can say that negative and positive charges are equal in magnitude and cancel the each other. For example if neutral atom has 6 protons than it must have 6 electrons. The sum of neutrons and protons is the mass number of an atom while the number of protons are number of electrons is the atomic number of an atom.
For example:
The mass number of carbon is 12 amu and it has six protons thus to find the number of neutrons we would subtract the 6 from 12 and get number of neutrons.
