Every decision is arbitrary in some way. We, as humans, defined it would be better to arrange the periodic table a certain way, based on characteristics of the elements, but the characteristics chosen are arbitrary. We could've just chosen a different set for ordering, like alphabetical.

Tho, that's a correct (IIRC) yet weird usage of "arbitrary", and since language exists to talk, not to be correct, we might as well not use this definition of arbitrary and stick to what conveys information better.

Edit: I'm wrong! perhaps "correct" was the word I meant to say, not sure tho.