# What is the number of neutrons in a nucleus?

Apr 30, 2017

Whatever you want, until you specify what nucleus you are talking about.

The quantity can be found by inspection of the mass number of an isotope and the atomic number. Suppose we examine a very specific nucleus, such as $\text{_(6)^(12) "C}$.

Recall that the superscript is the mass number $A$ and the subscript is the atomic number $Z$. The mass number is the sum of the neutrons and protons in the nucleus, while the atomic number IS the number of protons. That is,

$Z = 6$

A = Z + ?

= 6 + ? = 12

Hence, the number of neutrons in $\text{Carbon-12}$ is

? = 12 - 6 = bb(6).