[yummyfajitas]

They are both variations of carbon because they have the same electronic structure, i.e. the configuration of electrons about C-12 and C-13 is identical (or similarly H-1 and H-2).

I'll make a bad analogy now.

The electronic structure is like a set of hooks attached to the atom. Hydrogen has 1 free hook, carbon 4, and these hooks form chemical bonds. Hydrogen and deuterium have the same set of hooks, as do C-13 and C-14. But deuterium is heavier than hydrogen, and this makes it harder to unhook it when it attaches to another atom, even if the set of hooks is identical.

[13ren]

Sorry, that was a suggestion phrased as a question (i.e. I know what isotopes are). I was suggesting a name that signifies operational properties rather than "the" definition of what it is. If heavy water became commonly available, this would undoubtedly occur.

It's like features vs. benefits, which I've been working with over a few weeks, to understand the need for my product, and the gaps left by existing offers in the marketplace. Quite possibly, I'm thinking too much in those terms :-)