# Question #8d740

Dec 18, 2016

It is the difference between mass number $Z$ and the atomic number $A$. In a neutral atom the number of electrons is the same as the number of protons.
Example: Chlorine-35 has a mass number of $Z = 35$
From the periodic table we see that the atomic number $A = 17$, which is also the number of protons.
Now the number of neutrons $= Z - A = 35 - 17 = 18$