Surfaces of silicon, germanium, graphite (carbon) and germanium-silicide are active in EPR measurements. Mainly group 14 (formerly group IV) elements show EPR signals from a surface after crushing. Crystals of elements from groups 13 to 15 prefer to have the (110) plane exposed as a surface. On this surface, an atom of group 13 has 3/4 dangling bond, and an atom of group 15 has 5/4 dangling bond. Because of dehybridization of surface orbitals (caused by the decreased number of nearest neighbor atoms around the surface atom), a group 13 atom will have a largely unfilled dangling orbital since it has valence 3 and makes three bonds, while a group 15 atom will have a fully occupied dangling orbital at the surface. In that case, there is hardly any unpaired electron density, which results in a weak EPR signal for such materials. Clean cleaved surfaces of such materials form paired electron localized states on alternate sites resulting in a very weak to no EPR signal. Not well-cleaved surfaces and microcracks obtained from crushing, cleaving, abrading, neutron or high-energy ion irradiation or heating and rapid cooling in vacuum give a measurable EPR signal (a characteristic signal in Si at g = 2,0055). The presence of oxygen and hydrogen gas affects the EPR signal from microcracks by affecting the single electron spin centers. The gas molecules can get trapped and, when staying close to a spin center, affect the EPR signal. When a microcrack is sufficiently small, the wave functions of the dangling bond states extend beyond the surface and can overlap with wave functions from the opposite surface. This can create shear forces in the crystal surface, causing atom layers to realign while creating dangling bonds in the process.