8.2 Hybrid atomic orbitals  (Page 4/14)

sp ³ Hybridization

The valence orbitals of an atom surrounded by a tetrahedral arrangement of bonding pairs and lone pairs consist of a set of four sp ³ hybrid orbitals . The hybrids result from the mixing of one s orbital and all three p orbitals that produces four identical sp ³ hybrid orbitals ( [link] ). Each of these hybrid orbitals points toward a different corner of a tetrahedron.

A molecule of methane, CH ₄ , consists of a carbon atom surrounded by four hydrogen atoms at the corners of a tetrahedron. The carbon atom in methane exhibits sp ³ hybridization. We illustrate the orbitals and electron distribution in an isolated carbon atom and in the bonded atom in CH ₄ in [link] . The four valence electrons of the carbon atom are distributed equally in the hybrid orbitals, and each carbon electron pairs with a hydrogen electron when the C–H bonds form.

In a methane molecule, the 1 s orbital of each of the four hydrogen atoms overlaps with one of the four sp ³ orbitals of the carbon atom to form a sigma (σ) bond. This results in the formation of four strong, equivalent covalent bonds between the carbon atom and each of the hydrogen atoms to produce the methane molecule, CH ₄ .

The structure of ethane, C ₂ H _6, is similar to that of methane in that each carbon in ethane has four neighboring atoms arranged at the corners of a tetrahedron—three hydrogen atoms and one carbon atom ( [link] ). However, in ethane an sp ³ orbital of one carbon atom overlaps end to end with an sp ³ orbital of a second carbon atom to form a σ bond between the two carbon atoms. Each of the remaining sp ³ hybrid orbitals overlaps with an s orbital of a hydrogen atom to form carbon–hydrogen σ bonds. The structure and overall outline of the bonding orbitals of ethane are shown in [link] . The orientation of the two CH ₃ groups is not fixed relative to each other. Experimental evidence shows that rotation around σ bonds occurs easily.

An sp ³ hybrid orbital can also hold a lone pair of electrons. For example, the nitrogen atom in ammonia is surrounded by three bonding pairs and a lone pair of electrons directed to the four corners of a tetrahedron. The nitrogen atom is sp ³ hybridized with one hybrid orbital occupied by the lone pair.