Rules in Common with Regular Chess
In atomic chess, the initial set-up of the pieces is the same as in regular chess, and White moves first. All pieces move as they do in regular chess. Castling and en passant captures are executed as they are in regular chess. Draws by three-fold repetition of position, the 50-move rule, and by agreement apply as in regular chess. Pawn promotions apply just as in regular chess. Finally, as in regular chess, the object of atomic chess is to win your opponent's king (we'll discuss this below).
The Atomic Rule
The major rule difference between chess and atomic chess is the atomic rule, which states:
When a piece is captured, not only is the captured piece removed but so too is the capturing piece, as well as all non-pawn pieces within a one-square radius of the square on which the capture took place.
We gave an example in the previous chapter. Let's give another:
In this position, Black may capture the queen with his pawn. If he does so, then an explosion is said to have taken place centered at b6 with a one-square radius. Not only will the queen be removed, but so too will the a-pawn that's doing the capturing, as well as any other non-pawn pieces within the outlined squares. After the capture the position will look like this:
For en passant captures, the center of the explosion is at the square behind the pawn being captured. Consider the following diagram:
Here, Black has just moved his pawn to d5. White can capture the pawn with his own, en passant. If he does, the center of the explosion will be at d6, and the captured pawn, the capturing pawn, and all other non-pawn pieces in the outlined squares will be removed. The resulting position would be:
No Suicide Rule
A player cannot make a move that causes the explosion of one's own king. Consider:
Here, White cannot take the pawn with his rook to kill the enemy king because his own king would get killed in the process. So, draws like this are not legal.
Note that, due to this rule, a king can never capture another piece.
For further illustration of this rule, consider:
Winning by Explosion Rule
If you make a move that causes the explosion of your opponent's king (but not your own), then you win the game. This type of win happens frequently in speed games when a player isn't careful.
Consider:
Here, White can play Ne5 and threaten to capture on d7 and on f7, in either case, killing the Black king and winning the game. If White plays Ne5, Black has no defense.
Neighboring Kings Rule
Unlike in regular chess, the two kings are allowed to reside on adjacent (neighboring) squares. The logic behind this is that, since neither king can capture the other according to the no suicide rule, adjacent kings are not checking each other.
This rule has some interesting consequences. For example, consider:
Here, even though White is a queen up, the game is drawn. All Black has to do is keep his king adjacent to White's so that the Black king cannot be taken.
Atomic Check
A move that directly threatens the opponent's king (i.e. a check in normal chess) is considered a check in atomic chess as well except when the two kings are on adjacent squares (this is because the capture of one king would cause the indirect explosion of the other). Note that a move that threatens to explode the opponent's king indirectly (i.e. by taking a piece next to the king) is not considered atomic check. (Note: in practice, we simply say "check" instead of "atomic check.")
Consider:
White is in atomic check by the Black queen. He can get out of atomic check with one of six possible moves: Ke2, Qd2, c3, b4, Nxa6, or Bxa6. One of the latter two moves would be wisest.
Consider:
Here, White is not in atomic check, even though the Black bishop is threatening to explode the king by taking the knight. If it is White's move, it would be wise (but not required) for him to make one of the following moves: Kc1, Kc2, or g4.
Consider:
Here, White is not in atomic check even though the bishop is directly attacking the king. This is because the two kings are on neighboring squares.
Explosion Beats Check Rule
A player may not make a move that causes him to be in atomic check unless it is a move that causes the opponent's king to explode.
For example, consider:
In this diagram, it is illegal for White to play Nxf4 because doing so would cause an explosion, removing the Black knight on g4 in the process and exposing the White king to atomic check from the Black bishop.
However, it is important to remember that causing a king to explode takes precedence over atomic check.
Consider:
In this diagram, White has just moved his queen to e7, atomic checking the Black king. Black cannot get out of atomic check, but he is not in atomic checkmate. In fact, Black wins by playing his only legal move: Qxf2, causing the White king to explode.
Atomic Checkmate
If a player is in atomic check, then he must make a move that either removes the atomic check or explodes his opponent's king. If he can do neither of these, then he is in atomic checkmate and he loses. (Note: In practice, we simply say "checkmate" instead of "atomic checkmate.")
Consider:
In this diagram, Black is in atomic checkmate and he has lost. He is in atomic check by the White bishop on d7. He cannot take that bishop because it would cause an explosion killing his own king, there is nowhere for his king to move to, and he cannot cause the White king to explode.
Atomic Stalemate
If it is a player's turn, and he is not in atomic check and he has no legal moves (i.e. moves that do not explode his own king and do not place himself in atomic check), then he is in atomic stalemate and the game is a draw. (Note: in practice, we simply say "stalemate" instead of "atomic stalemate.")
For example, consider:
In this diagram, if it is Black's turn, he is in atomic stalemate and the game is drawn. He is not allowed to take any pieces, and he cannot move to either of the open squares f1 or f2 because that would place him in atomic check. Note further that Black is not currently in atomic check because the kings are neighboring.
Castling
The castling move is executed as in normal chess. As in normal chess, it is illegal to castle if your king has previously moved or if your rook with which you are castling has previously moved. It is also illegal to castle if you are currently in atomic check, if moving the king one square closer to the rook (with the rook staying put) would place you in atomic check (i.e. you cannot castle through check), or if completing castling would place you in atomic check.
For example, consider:
In this diagram White is allowed to castle long if he has never previously moved his king or rook. This is because he is not currently in atomic check (the kings are neighboring), moving to d1 would not place him in atomic check, and completing the castling move would not place him in atomic check.
Final Words
That's all the rules. You now know how to play a game of atomic chess. Keep in mind that the rules are really simpler than the length of this chapter may make them seem. Basically, you try to kill your enemy's king indirectly by explosion or directly by checkmate. Also, it is very important to remember that if you are in check and you can indirectly explode your opponent's king, then do so! Oh, and go right ahead and move your king next to your opponent's if you wish. All the rules about check, checkmate, and stalemate are there mostly in order to understand exactly when castling is possible.