Take note that the nucleus of an atom is composed of protons and neutrons. And the number of particles present in the nucleus is referred as mass number (Also, called as atomic mass).
mass number = Number of Protons + Number of Neutrons
So, to determine the number of neutrons in atom, we only have to subtract the number of protons from the mass number.
# of neutrons = Mass Number - Number of Protons
To be able to apply this formula, refer to the atomic number and atomic weight of the elements. These two are listed in the Periodic Table of Elements.
Take note that atomic number refers to the number of protons in an atom of an element.
# of protons = atomic number
While, atomic weight is weighted average of the atomic masses of all the natural isotopes of an element. Its values in the Periodic Table are in decimal form. So to get the atomic mass, round off the atomic weight to its nearest whole number.
mass number = atomic weight rounded off to nearest whole number
For example, sodium's atomic number is 11 and its atomic weight is 22.989. So, sodium contains 11 protons and its mass number is 23.
Applying the formula above, the number of neutrons will be:
# number of neutrons = Mass Number - Number of Protons
# number of neutrons = 23 - 11
# number of neutrons = 12
Hence, there are are 12 neutrons in an atom of sodium.