# How many neutrons are found in the unstable isotope phosphorus-36?

The atomic number, $Z$, of phosphorus is $15$. Thus, there MUST be 21 neutrons in ""^36P.
Atomic mass is ALWAYS determined by the number of nuclides, massive nuclear particles (protons or neutrons). Of course, the number of protons ($= Z$) determines atomic identity. Thus, I know for certain that this isotope has $15$ fundamental, positively charged massive particles in its nucleus. Why? Because it is a phosphorus atom, for which $Z$ (the atomic number) $=$ $15$.