You can drop atoms to any square that does not contain enemy atoms. Center squares can hold 3 atoms, borders 2 and corners only one atom. Dropping more atomes will cause an exlosion moving the atoms to adjacent squares and changing color of prior stones in the square. The moving stones may start new explosions.
 
You lose when you have no atoms left.
 
You should use Expert and over a minute think time to get some resistance.