Step-by-step explanation:
Atomic number is defined as the sum of total number of protons present in an atom. Each element has different atomic number.
Whereas mass number is the sum of total number of both protons and neutrons present in an element.
For example, an atom with mass number as 15 and has atomic number 7. Then it means it has 7 protons.
Therefore, number of neutrons present in it will be calculated as follows.
Mass number = no. of protons + no. of neutrons
15 = 7 + no. of neutrons
no. of neutrons = 15 - 7
= 8
Thus, we can conclude that the number of neutrons is calculated as mass number minus total number of protons present in the atom.