Due to the reactivity of dangling bonds, the semiconductor native oxide will form due to adsorption of gas molecules, the only remaining dangling bonds are located at oxygen vacancies. Dangling bonds form an sp -hybridized bond with the adsorbed molecule, which have a metallic character. They are often the only defect sites present on atomic semiconductors, which provide such "soft centers" for molecules to adsorb to. When no gas adsorption is possible (for example for clean surfaces in vacuum), the surface energy can be reduced by reorganizing bonding electrons, creating lattice strain in the process. In case of the (001) surface plane of silicon, a single dangling bond on each atom will be formed, while pairing the other electron with a neighboring atom. Removal of dangling bond surface states on the silicon (001) surface from the band gap can be achieved by treatment of the surface with a monolayer of selenium (alternatively, sulfur was proposed). Selenium can attach to the silicon (001) surface and can bind to surface dangling bonds, bridging between silicon atoms. This releases the strain in the silicon surface and terminates the dangling bonds, covering them from the outside environment. When exposed, dangling bonds can act as surface states in electronic processes.