Each player takes it in turns to place an atom on the grid. Players can place additional atoms onto their own atoms.
If too many atoms are place on the same tile the atom goes critical and will explode and place additional atoms. When the atom goes critical it will send additional atoms horizontally and vertically away from it. If this atom lands on a square that has another atom on it, it will add to that atom. Also if the atom is another players it will be taken over by the atom that went critical.