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 and epitaxy, when describing single atoms lying on surfaces and surface roughness. The word is a portmanteau of "adsorbed atom". A single atom, a cluster of atoms, or a molecule or cluster of molecules may all be referred to by the general term "adparticle". This is often a thermodynamically unfavorable state. However, cases such as graphene may provide counter-examples.

″Adatom″ is a portmanteau word, short for adsorbed atom. When the atom arrives at a crystal surface, it is adsorbed by the periodic potential of the crystal, thus becoming an adatom. The minima of this potential form a network of adsorption sites on the surface. There are different types of adsorption sites. Each of these sites corresponds to a different structure of the surface. There are five different types of adsorption sites, which are: on a terrace, where the adsorption site is on top of the surface layer that is growing; at the step edge, which is next to the growing layer; in the kink of a growing layer; in the step edge of a growing layer, and in the surface layer, where the adsorption site is inside the lower layer.

Out of these adsorption site types, kink sites play the most important role in crystal growth. Kink density is a major factor of growth kinetics. Attachment of an atom to the kink site, or removal of the atom from the kink, does not change the free surface energy of the crystal, since the number of broken bonds does not change. This gives that the chemical potential of an atom in the kink site is equal to that of the crystal, which means that the kink site is the one adsorption site type where an adatom becomes a part of the crystal.

If crystallography is used, or if the growth temperatures are higher, which would give an entropy effect, the crystal surface becomes rough, causing greater number of kinks. This means that adatoms have a greater chance of arriving at a kink site, to become part of the crystal. This is the normal mechanism of growth.

The opposite, so with a lower growth temperature, would give a smooth surface, which means that there is a higher number of terrace adsorption sites. There are still kink sites, but these are only found at the edges of steps. The crystal only grows through "lateral motion of the steps". This type of growth is called the layer mechanism of growth. How the adatoms grow on the surface depends on what interaction is the strongest or what the surface looks like. If the adatom-adatom interaction is the strongest, adatoms are more likely to create pyramids of adatoms on the surface. If the adatom-surface interaction is the strongest, the adatoms are more likely to arrange themselves in such a way as to create layers on the surface. But it also depends on the origins of the steps on the surface. In total there are five different types of layer growth: normal growth, step-flow growth, layer-by-layer growth, multilayer (or three-dimensional island) growth, and spiral growth.

Step-flow growth is observed on stair-like surfaces. These surfaces have a geometry with vicinal steps separated by "atomically flat low-index terraces". When adatoms attach to the edges of the steps, they move along the surface, until they find a kink site to attach to become part of the crystal. However, if the kink density is not high enough, and thus not all adatoms arrive at one of the kinks, additional steps, as if there is a flat surface with small two-dimensional islands on it, are created on the terraces, leading to a mixed growth mode, which leads to a change in layer growth type, from step-flow to layer-by-layer growth.

In layer-by-layer growth, the adatom-surface interaction is the strongest. A new layer is created through 2D islands, which is created on the surface. The islands grow until they spread out over the entire surface, and the next layer will start to grow. This growth is named Frank–Van der Merwe growth.

In some cases the cycle of making new layers in layer-by-layer growth is broken by kinetic constraints. In these cases, growth in higher layers starts before lower layers are finished, which means three-dimensional islands are created. A new type of growth, called multilayer growth, is started, instead of the layer-by-layer growth. Multilayer growth can be divided into Volmer-Weber growth and Stranski-Krastanov growth.