An adatom is an atom that lies on a crystal surface, and can be thought of as the opposite of a surface vacancy. This term is used in surface chemistry, when describing single atoms lying on surfaces and surface roughness. The word is a contraction of "adsorbed atom". A single atom, cluster of atoms, or a molecule or cluster of molecules may all be referred to by the general term "adparticle". This is a thermodynamically unfavorable state.