These puzzles are supposed to have one unique solution. If 2 or more clues can be interchanged without redefining the solution, then it cannot be unique.

4 x Atomic numbers are prime & symbols begin with "A".
Silver, Aluminium, Actinium & Gold

2 x Symbols have single letter & atomic numbers are prime.
Nitrogen & Potassium

Took some time to check, and I can see at least 16 solutions for this puzzle.
Even if you worked out what the groups were, you could be here all day getting the combination submitted.

There is also the question of "What is radioactivity?" Most substances decay over time and, if I remember my science classes, all 16 could be considered radioactive.

Great grid! I couldn't find the connections due to the many possibilities, but I got the 'A' and 'one letter symbol' category. I don't think it is too bad having so many connections... It is sort of the point of the game... And you could say all the element decay, but it is quite clear that elements such as Uranium are very radioactive, so the category works well, I think.
Overall, not as bad as Baffled makes it seem (well, in my opinion anyway)...