Carbon germanium—phosphorus bonds

The direct linking of like atoms, such an essential feature of carbon chemistry, occurs to only a limited extent with other elements. Metal-metal bonds are not uncommon but they are always weak examples are found in the polynuclear carbonyls (p. 306) and compounds such as SugRg and PbgRfi (R = alkyl). Among the other non-metals catenation is displayed by boron, silicon, germanium, phosphorus and sulphur none of the substances... 

Semiconductors such as silicon (Si), with only medium sized energy gaps, can easily be made into fairly good conductors. As the reader might remember from chemistry courses, silicon has a valence of four, similar to carbon, germanium, and tin. If an element such as phosphorus (P) or arsenic (As), with a valence of 5, is somehow put into a silicon crystal, quantum mechanics predicts that it will take on the valence of the rest of the crystal, so it only makes 4 bonds to the surrounding Si atoms. This leaves one extra electron in its outer shell, which is not used in any bonds, and is therefore easily taken away. 

Catenation is defined as the self-linking of an element to form chains and rings. Carbon, then, given the above discussion, is the all-time champion catenator, much better than silicon (or sulfur, boron, phosphorus, germanium, and tin, the other elements that show this ability). Why should this be so A comparison of the relevant carbon and silicon bond energies as shown below is helpful ... 

Similar arguments apply to impurity states. Any impurity which is bonded with its optimum valency is expected to form a part of the ideal network and contribute only to the conduction and valence bands. Oxygen, nitrogen, carbon, and germanium all behave in this way, forming alloys with a-Si H. Most of the phosphorus and boron atoms which are added as dopants, are in three-fold coordinated inactive sites... 

The chemical reactions of nitrogen and phosphorus are similar because they share the same number of electrons in their outer shell (five). The reactivity of oxygen resembles the reactivity of sulfur because of their shared outer-shell occupancy (six). This outer-shell occupancy of an atom is called its valence. Carbon has a valence of four (with four electrons in its outer shell), and its chemistry shares some similarities with silicon, which also has a valence of four. Silicon, germanium, tin, and lead, which have the same valence, have all been used in various proportions to form semiconductors, interesting and important materials that we will investigate later when we discuss chemical bonding. 

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