Question

How would you figure the number of neutrons in an atom?

Answer 1

The mass of an atom is the number of protons and neutrons in it (electrons are light enough to be ignored). To find the number of neutrons in the atom, subtract the number of protons from its mass (you can find its mass in a periodic table). The result is your answer.

Answer 2

Mass Number = (Atomic Number) + (Number of Neutrons) so you solve for the Number of Neutrons and you get:
Number of Neutrons = (Mass number) - (Atomic Number) 

Mass Number equals protons plus neutrons, round atomic weight to nearest whole number
Atomic Number equals number of Protons