When islands are formed of atoms adsorbed on the surface, small islands are generally unstable and often break up into individual atoms. However, with increasing size (with the size meaning the number of atoms forming the island) the stability of the island increases, and after passing a critical size the probability of growth of the island exceeds the probability of its breakup. For example, if the smallest stable formation is a dimer then = 1; if it is a trimer, then = 2, etc.