The explanation of university chemistry is that the energy of surface molecules is greater than that of internal molecules, which is called surface energy.
Surface energy level: the original three-dimensional periodicity of crystals is interrupted on the surface to form the electronic energy level, also known as surface state. Because the surface potential field of electrons in the surface layer is different from that in three-dimensional crystals, the electronic States show special properties.
Theoretically, the traditional energy band calculation method (see energy band theory) is often applied to the calculation of electron energy and wave function on the surface thin layer. The wave function describing the surface electronic state only exists in the area of several atomic layers on the surface, and the wave function decays rapidly when it goes deep into the interior. From the chemical bond model, the surface energy level comes from the unsaturated valence bond of the surface atoms, which is called hanging bond.