sp³ Hybridization

An atom surrounded by a tetrahedral arrangement of bonding pairs and lone pairs has a set of four sp3 hybrid orbitals. The hybrids result from the mixing of one s orbital and all three p orbitals. This mixing results in four identical sp3 hybrid orbitals, as illustrated below. Each of these hybrid orbitals points toward a different corner of a tetrahedron.

The hybridization of an s orbital (blue) and three p orbitals (red) produces four equivalent sp3 hybridized orbitals (yellow) oriented at 109.5° with respect to each other.

A molecule of methane, CH4, consists of a carbon atom surrounded by four hydrogen atoms. These hydrogen molecules are positioned at the corners of a tetrahedron. In the case of methane, the carbon atom in methane exhibits sp3 hybridization. Figure 2 below shows the orbitals and electron distribution in both an isolated carbon atom and in the bonded atom in CH4 molecule. The carbon atom has four valence electrons that distribute equally in the hybrid orbitals. Each carbon electron pairs with a hydrogen electron when forming the C–H bonds.

The four valence atomic orbitals from an isolated carbon atom all hybridize when the carbon bonds in a molecule like CH4 with four regions of electron density. This creates four equivalent sp3 hybridized orbitals. Overlap of each of the hybrid orbitals with a hydrogen orbital creates a C–H σ bond.

In a methane molecule, the 1s orbital of each of the four hydrogen atoms overlaps with one of the four sp3 orbitals of the carbon atom. This overlap creates a sigma (σ) bond between the carbon and each hydrogen atom creating four strong, equivalent covalent bonds. As a result, the carbon atom forms a stable methane molecule, CH4, with each hydrogen atom.

Hybridization in Ethane and Other Molecules

The structure of ethane, C2H6, resembles that of methane, with each carbon atom surrounded by four neighboring atoms arranged in a tetrahedral shape—three hydrogen atoms and one carbon atom ([link]). However, in ethane an sp3 orbital of one carbon atom overlaps end to end with an sp3 orbital of a second carbon atom to form a σ bond between the two carbon atoms. Each of the remaining sp3 hybrid orbitals overlaps with an s orbital of a hydrogen atom. This overlap creates carbon–hydrogen σ bonds. [link] shows the structure and overall outline of the bonding orbitals in ethane. The orientation of the two CH3 groups can vary relative to each other. Experimental evidence shows that rotation around σ bonds occurs easily.

(a) In the ethane molecule, C2H6, each carbon has four sp3 orbitals. (b) These four orbitals overlap to form seven σ bonds.

An sp3 hybrid orbital can also hold a lone pair of electrons. For example, the nitrogen atom in ammonia has three bonding pairs and a lone pair of electrons directed to the four corners of a tetrahedron. The nitrogen atom is sp3 hybridized with one hybrid orbital occupied by the lone pair.

The molecular structure of water is consistent with a tetrahedral arrangement of two lone pairs and two bonding pairs of electrons. Therefore, we say that the oxygen atom is sp3 hybridized. In this configuration, two of the hybrid orbitals occupied by lone pairs and two by bonding pairs. Since lone pairs occupy more space than bonding pairs, structures that contain lone pairs exhibit slight distortions in bond angles. Perfect tetrahedra have angles of 109.5°, but the observed angles in ammonia (107.3°) and water (104.5°) are slightly smaller. Other examples of sp3 hybridization include CCl4, PCl3, and NCl3.