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In chemistry, tetravalence is the state of an atom with four valence electrons available for covalent chemical bonding in its outermost electron shell, giving the atom a chemical valence of four. An example is methane (CH4): the tetravalent carbon atom forms a covalent bond with four hydrogen atoms. The carbon atom is called tetravalent because it forms 4 covalent bonds. A carbon atom has a total of six electrons occupying the first two shells, i.e., the K-shell has two electrons and the L-shell has four electrons. This distribution indicates that in the outermost shell there are one completely filled 's' orbital and two half-filled 'p' orbitals, showing carbon to be a divalent atom. But in actuality, carbon displays tetravalency in the combined state. Therefore, a carbon atom has four valence electrons. It could gain four electrons to form the C4− anion or lose four electrons to form the C4+ cation. Both these conditions would take carbon far away from achieving stability by the octet rule. To overcome this problem carbon undergoes bonding by sharing its valence electrons. This allows it to be covalently bonded to one, two, three or four carbon atoms or atoms of other elements or groups of atoms. Let us see how carbon forms the single, double and triple bonds in the following examples.
A carbon atom has four electrons in its outermost valence shell. So, it needs four more electrons to complete its octet. A carbon atom completes its octet only by sharing its valence electrons with other atoms. As a result, a carbon atom forms four covalent bonds by sharing valence electrons with other atoms. This is known as tetravalency of carbon ("tetra" means four). These four valences of carbon are directed towards four corners of a tetrahedron, and inclined to each other at an angle of a 109° 28´.
The carbon atom is assumed to be at the center of the tetrahedron. In common use, the four valences of carbon are shown by four bonds around a carbon atom as shown alongside
Methane molecule: Each carbon atom has four electrons in its outermost shell. Thus, it requires four more electrons to acquire a stable noble gas configuration. Each of the hydrogen atoms has only one electron in its outermost shell and requires one more electron to complete its outermost shell (to acquire He configuration). To achieve this, one carbon atom forms four single covalent bonds with four hydrogen atoms.
Carbon dioxide molecule: Each carbon atom has four electrons in its outermost shell and each oxygen atom has six electrons in its outermost shell. Thus, each carbon atom requires four, and each oxygen atom requires two more electrons to acquire noble gas configurations. To achieve this, two oxygen atoms form a double covalent bond with carbon.
Acetylene molecule: Each carbon atom has four electrons in its outermost shell and each hydrogen atom has only one electron in its outermost shell. Two carbon atoms share two electrons each with hydrogen atoms to form single bonds. Each carbon then requires three more electrons to acquire a stable configuration of the nearest noble gas (neon). This is done by mutually sharing three pairs of electrons between the two carbon atoms to form a triple bond.