# How many neutrons are in ""^239 Pu?

There are $145$ neutrons in the ""^239Pu nucleus.
You specified elemental plutonium. Thus we know for a fact that $Z$ $=$ $94$. Since we have the mass number specified by the isotope notation, there are $239 - 94$ massive, neutral particles in the plutonium nucleus.
To what does $Z$ refer?