It'll work fine until you find you want to keep track of U-235 and U-238 separately.
What constitutes identity is context-dependent. Yeah, on one level they're the same thing. On another they aren't.
Surely such a system would have a products table with something like nominal element fk, purity, mass columns anyway. (And stock table with fk to products.)
Then nothing wrong with 'natural' key on the elements, but sure, products are the thing you've (personally) made up, so of course there's no 'someone else's' key to use.
Also it gets fun if you want to store anti-particles too.