# How do we find the number of neutrons in the ""^31P isotope?

Simply subtract $15$ from $31$:
$31 - 15 = \text{number of neutrons in the phosphorus isotope} = 16$
For the ""_15^31P isotope, $\text{Z, the atomic number}$, the subscript, is superfluous. The element is phosphorus, and $Z$, $\text{the atomic number}$ $=$ $15$ BY DEFINITION.
The $\text{mass number}$, the superscript, is the number of massive particles, protons and neutrons, contained in the nucleus of the isotope, and this is given by the superscript. This is something you really should get your head round. Hydrogen has 3 common isotopes, $\text{^1H, "protium", ""^2H, "deuterium} ,$ and ${\text{tritium",}}^{3} H .$ Can you tell me the number of neutrons each nucleus contains?