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 them. Atoms that possess too few bonding partners to satisfy their valences and that 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". A dangling bond in (bulk) crystalline silicon is often pictured as a single unbound hybrid sp orbital on the silicon atom, with the other three sp orbitals facing away from the unbound orbital. In reality, the dangling bond unbound orbital is better described by having more than half of the dangling bond wave function localized on the silicon nucleus, with delocalized electron density around the three bonding orbitals, comparable to a p-orbital with more electron density localized on the silicon nucleus. The three remaining bonds tend to shift to a more planar configuration. It has also been found in experiments that Electron Paramagnetic Resonance (EPR) spectra of amorphous hydrogenated silicon (a-Si:H) do not differ significantly from the deuterated counterpart, a-Si:D, suggesting that there is hardly any backbonding to the silicon from hydrogen on a dangling bond. It also appeared that the Si-Si and Si-H bonds are about equally strong.