Each player in turn places an atom in a cell on the board. They can either place it in an empty cell, or any cell with their own colour atoms in it. If the placed atom causes the cell to be overloaded, the cell will explode with the atoms moving out into all orthogonally adjacent cells, turning any existing atoms in those cells into the player's colour. Chain reactions can occur after the initial explosion, which can cause sweeping changes across the board.
The maximum number of atoms a cell can hold before becoming overloaded is:
Once a player's last atom has been wiped out, they are out of the game. The winner is the last surviving player.
All source code for this implementation of Atoms can be found in the atoms-www GitHub project.