What are the different forms called in an element that has forms with different numbers of neutrons?
As far as we know most matter in this universe is hydrogen... And this is composed atom with ONE defining proton in is nucleus, about which a single electron, of charge opposite to the proton, whizzes about. And we represent the most common isotope of this species as
The nuclear core CAN contain another massive particle, the neutron, and such an atom we would represent as
If you look up the atomic masses on the Periodic Table, you find that most elements have non-integral atomic masses, given that they are calculated on the basis of an envelope of isotopes of DIFFERENT abundance...
e.g. iron metal...