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

Jul 1, 2018

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, $\text{protium}$...

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, $\text{deuterium isotope}$. And the presence of another neutron in the nucleus gives ""^3H, i.e. $\text{the tritium isotope...}$

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, $F e , 55.845 \left(5\right) \cdot g \cdot m o {l}^{-} 1$..