Each side begins with 11 atoms off-board. On the first turn White drops an atom on any square. Then players alternate placing atoms on any empty square which is adjacent to exactly one other atom (of either color). After all atoms are placed they form one large molecule, which may split into several smaller molecules during the game.
 
Each atom has a valence, which is the number of atoms (of either color) adjacent to it. Players must now move one of their atoms to any adjacent empty square provided that its valence in the new position is exactly the same as its valence in the old position.
 
Any atom which is not adjacent to an enemy atom is unstable. Immediately after each move the computer will automatically remove all unstable atoms from the board. This also occurs after Black makes his last drop.
 
A player wins by stalemating his opponent, (leaving him with no legal moves). A player also wins by leaving an empty board (after the removal operation); in fact this is a special case of stalemate since his opponent would have no moves at all.
 
 
Isotopes Variant: (In physics an isotope is a variant form of an atom.) In Isotopes an atom can move through any number of empty squares in any orthogonal or diagonal direction stopping on any empty square provided that its valence in the new position is exactly the same as its valence in the old position.
 
Radioactive Variants: In these variants there is an additional suicide move: In Radioactive Atoms an atom may also move to any empty adjacent square which is not adjacent to any other atoms. In Radioactive Isotopes an atom may also move through any number of empty squares in any orthogonal or diagonal direction stopping on any empty square which is not adjacent to any other atoms.
 
Geiger-Counter Variants: These variants have a count-condition. The game ends when the player on the move has no legal move. The winner is the player having the most atoms left on the board. If the atom counts for both players are equal, it is a draw.