Question

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

Answer 1

The atomic number, `Z`, of phosphorus is `15`. Thus, there MUST be 21 neutrons in `""^36P`.

Explanation:

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`.

To account for the given atomic mass, there must be 21 neutrons in the nucleus. This is possibly a very short-lived isotope. Nucular chemistry and physics are not my strong suites.