How many neutrons are there in a phosphorus atom?

Question

I believe the answer is b. 16

a. 15
b. 16
c. 31.
d. none

Answer 1

The number of neutrons is:
(mass number) - (atomic number)
The mass number of the most common isotope of phosphorus is 31 (the rounded off atomic mass). I will let you look up the atomic number and do the subtraction.

Answer 2

31 - 15 =16 so the answer is 16