To calculate the number of neutrons in an atom, you need to subtract the number of protons (atomic number) from the mass number.

Given isotope: 32P

Mass number = 32

Atomic number (p) = 15

Number of neutrons (n) = Mass number - Atomic number

n = 32 - 15

n = 17

Therefore, the number of neutrons in an atom of 32P is 17.