In order to gain enough electrons to fill their valence shells (see also octet rule), many atoms will form covalent bonds with other atoms. In the simplest case, that of a single bond, two atoms each contribute one unpaired electron, and the resulting pair of electrons is shared between both atoms. Atoms which possess too few bonding partners to satisfy their valences and which possess unpaired electrons are termed free radicals; so, often, are molecules containing such atoms. When a free radical exists in an immobilized environment, for example, a solid, it is referred to as an "immobilized free radical" or a "dangling bond".
Free and immobilized radicals display very different chemical characteristics from atoms and molecules containing only complete bonds. Generally, they are extremely reactive. Immobilized free radicals, like their mobile counterparts, are highly unstable, but gain some kinetic stability because of limited mobility and steric hindrance. While free radicals are usually short lived, immobilized free radicals often exhibit a longer lifetime because of this reduction in reactivity.
Some allotropes of silicon, such as amorphous silicon, display a high concentration of dangling bonds. Besides being of fundamental interest, these dangling bonds are important in modern semiconductor device operation. Hydrogen introduced to the silicon during the synthesis process is well known to replace dangling bonds, as are other elements such as oxygen.
In computational chemistry, a dangling bond is an error in structure creation, in which an atom is inadvertently drawn with too few bonding partners, or a bond is mistakenly drawn with an atom at only one end.
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