Rules of Atomic Chess
Atomic chess is played similarly to normal chess, with the major difference being how pieces are captured. Pieces around captured square explode. The goal in atomic chess is to either explode or checkmate the opponent's king.
In atomic chess after a capture not only is the captured piece removed, but also the capturing piece, as well as any non-pawn pieces adjacent to the capture square (horizontally, vertically, and diagonally). The pieces adjacent to the capture square are known as having been exploded.
As an example, in the above position if white where to play fxg4, the knight on g4 is captured and removed, the pawn making the capture is removed. Any non-pawn pieces left on squares adjacent to g4 (namely f5,g5,h5,f4,h4,f3,g3,h3) will explode and be removed, in this case the white bishop on h3 is removed.
Captures - Kings
In atomic chess it is illigal for kings to capture opponent's pieces. It is also illigal to make any captures that would explode one's own king. However, since kings cannot attack, it is legal for the kings to occupy adjacent squares. This is known as connected kings, and neither king is in direct check by other pieces.
Direct checks in atomic are the same as in normal chess, i.e. when a king is under attack by one or two of the opponent's pieces. Unlike normal chess, in atomic one additional legal response to a direct check is to immediately explode the opponent's king, thus ending the game.
In the above position capture of the white queen on f8 would result in the explosion of the king, and is thus illigal. The black king is in check and cannot move to either e7 or f7, because those squares are under attack by the white queen. The only legal response in the above position is to capture with the bishop on d2, exploding the white king, and winning the game for the player with the black pieces.
Indirect check is any move that attacks an opponents piece next to their king, threatening to explode the king on the next turn.