Question

Phosphorus has an atomic mass of 31 and an atomic number of 15, so the number of neutrons must be _______________.

Answer 1

Remember this:

atomic mass= atomic number + number of neutrons 

The atomic number (same thing as number of protons) in this case is 15. The atomic mass is 31. 

So, we subtract 15 from 31 to find the number of neutrons.

31-15=16 

16 neutrons!!!