Meta Hint 1
I like to imagine the black squares in this puzzle are little nuclear physicists
Meta Hint 2
7 elements are in the lab, being split (fission, hehehe) by our crack team of physicists
Meta Hint 3
14 extra letters across 14 different clues. If you're stuck here, I'd wager you're overthinking their arrangement
Meta Hint 4
Put each extra letter in order according to its clue's first letter