In physical chemistry, a hot atom is an atom that has a high kinetic or internal energy. When molecule AB adsorbs on a surface dissociatively, 1. * both A and B adsorb on the surface, or 2. * only A adsorbs on the surface, and B desorbs from the surface. In case 2, B gains a high translational energy from the adsorption energy of A, and hot atom B is generated. For example, the hydrogen molecule, because of its light mass, gets a high translational energy. Such a hot atom does not fly into vacuum but is trapped on the surface, where it diffuses with high energy.