How can you calculate the number of neutrons in an atom?

Number of neutron can be simply calculated by:

Number of neutron=mass number-atomic number

As;

Mass number is the sum of number of proton and neutron in an atom

And

Atomic number is the number of proton in an atom.

If we don't know the mass or atomic number of an atom.we can got it from periodic table in which mass number is given on top and atomic number in bottom.

For example

In sodium; the mass number is 23

Atomic number is 11

By applying formula

No.of neutron=23-11=12