No Laughing Matter

Normal sudoku rules apply. Digits on arrows sum to the digit in the connected circle. Digits in cages sum to the small clue in the top left corner.

Nitrous Oxide (N2O) -- commonly known as Laughing Gas -- is composed of Nitrogen (atomic number 7) and Oxygen (atomic number 8).

There is one 7-8 domino (a Nitrous Oxide molecule?) in the central box; no other molecules exist. That is to say, no other domino or diagonal pair contain a 7-8 combination (but, 7-7 or 8-8 diagonals are allowed).

Of the remaining 7’s and 8’s (“free atoms”), none of them are adjacent to the central molecule (orthogonally or diagonally) and none of them are in cages or on circles/arrows.