Explain why it isn't possible for an atom to have a mass number of 10 and an atomic number of 12.

A question for my class, I totally forgot :(

Mass number = A = number of protons + number of neutrons
Atomic number = Z = number of protons

So with A = 10 and Z = 12, we would have 12 protons and minus two neutrons. It's not possible to have a negative number of neutrons.

Hydrogen has mass no of 1 and atomic no of 1 for all other elements mass no >atomic no. read the definitions of both.