Question

What are the different forms called in an element that has forms with different numbers of neutrons?

Answer 1

You speak of isotopes....and let us consider one simple example....

Explanation:

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 `""^1H`, `"protium"`...

The nuclear core CAN contain another massive particle, the neutron, and such an atom we would represent as `""^2H`, i.e. the deuterium isotope...the which has approx. 1% abundance. This is still hydrogen, (because `Z=1`), but it is TWICE as massive than garden variety hydrogen. And this is the `""^2H`, `"deuterium isotope"`. And the presence of another neutron in the nucleus gives `""^3H`, i.e. `"the tritium isotope..."`

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... `5.845%; ""^54Fe;` `91.754%; ""^56Fe;` `2.119%, ""^57Fe;` `0.282%, ""^58Fe.` The weighted average of these isotopic masses gives the atomic mass we read on the Periodic Table, `Fe, 55.845(5)*g*mol^-1`..