Critical Nucleus

A critical nucleus is the smallest cluster of atoms or molecules that can stabilize and grow into a new phase or structure during a phase change, like the formation of a liquid droplet from vapor or a solid crystal from a solution. It must reach a certain size; smaller clusters tend to dissolve back into the original phase because the energy cost of forming their surface outweighs the energy gained from the new phase. Once this critical size is achieved, the nucleus becomes stable enough to grow spontaneously, leading to the complete transformation.