# Zirconium has an atomic number of 40. How many neutrons are there in an isotope of zirconium-92?

In ""^92Zr there are NECESSARILY $52$ neutrons.
The sum of protons and neutrons gives the isotopic mass. For $Z r$, $Z = 40$ by definition. And thus, $92 = 40 + \text{number of neutrons}$.
https://en.wikipedia.org/wiki/Zirconium gives details of the individual isotopes. This isotope is about 20% abundant. The most common isotope is ""^90Zr. How many neutrons does this beast have?