D10.5 Types of Hybrid Orbitals

Formation of Hybrid Orbitals

Valence bond theory uses the extent of orbital overlap to infer the strengths of chemical bonds: greater overlap leads to bonds that are stronger and hence a molecule that is more stable. For a given atom in a molecule, overlap with orbitals on other atoms can be greater when some or all of the atom’s valence atomic orbitals (AOs) form hybrid orbitals. Hybrid orbitals are combinations of valence atomic orbitals that emphasize concentration of electron density in specific directions. A hybrid orbital’s greater electron density in a specific direction provides greater overlap with an orbital from another atom when forming a σ bond.

For an example of how orbital hybridization works, consider combining a single 2s AO with a single 2p AO, both on the same atom (Figure below). The 2s AO is spherically symmetric, so it has the same phase (mathematical sign) on either side of the nucleus, but the 2p AO changes sign at the nucleus. Thus, on one side of the nucleus, the 2s and 2p AOs are in phase, while on the other side they are out of phase.

Figure: sp Hybrid Orbitals.. One s orbital and one p orbital can form two sp hybrid orbitals. When the 2s AO and the 2p AO on the same atom are added (top), the sp hybrid orbital that is formed points toward the right—the region where both AOs have positive (blue) phase. When the 2p AO is subtracted from the 2s AO (bottom), the sp hybrid orbital formed points toward the left—the side where the two wave functions have opposite phase.

If we add the two AOs, the new hybrid orbital will be larger on the side where the AOs are in phase and smaller on the other side where the AOs are out of phase. If we subtract them, the resultant hybrid orbital will be larger on the side where the AOs are out of phase and smaller where they are in phase. Hence, from one 2s AO and one 2p AO, we can derive two sp hybrid orbitals.

Activity: Orbital Hybridization

sp Hybrid Orbitals

Combining the valence s AO with one of the valence p AOs yields two degenerate sp hybrid orbitals, as shown in Figure: sp Hybrid Orbitals for the case of 2s and 2p AOs. The two sp hybrid orbitals are oriented at 180° to each other—a linear geometry. After the hybridization, there are two unhybridized p AOs left on the atom.

Because these hybrid orbitals are formed from one s AO and one p AO, they have a 1:1 ratio of “s” and “p” characteristics, hence the name “sp”. One of the ways in which the hybrid orbitals exhibit their mixed “s” and “p” characteristics is in their energy. Specifically, the sp hybrid orbitals’ relative energies are about half-way between the s and p AOs from which they are derived, as illustrated in Figure: sp Hybrid Orbitals.

Figure: sp Hybrid Orbitals. Hybridization of the 2s and one of the 2p AOs forms two sp hybrid orbitals, oriented 180° with respect to each other; the two other 2p AOs remain unhybridized. (Move the slider to see the before/after of hybridization).

The hybridized orbitals are not energetically favorable for an isolated atom. For example, a beryllium atom is lower in energy with its two valence electrons in the 2s AO than if the electrons were in the two sp hybrid orbitals. However, in a covalent molecule, the one large lobe of each sp hybrid orbital gives greater overlap with another orbital from another atom, yielding lower energy σ bonds.

sp2 Hybrid Orbitals

Combining one valence s AO and two valence p AOs produces three degenerate sp2 hybrid orbitals, as shown in Figure: sp2 Hybrid Orbitals for the case of 2s and 2p AOs. The three sp2 hybrid orbitals are oriented at 120° with respect to each other and are in the same plane—a trigonal planar (or triangular planar) geometry. Each hybrid orbital is pointed towards a different corner of an equilateral triangle. After hybridization, there is one unhybridized p AO left on the atom.

The sp2 hybrid orbitals have twice as much “p” character as “s” character; this is indicated by the superscript “2” in sp2. Energetically, sp2 hybrid orbitals lie closer to the p AO than the s AO, as illustrated in Figure: sp2 Hybrid Orbitals (the sp2 hybrid orbitals are higher in energy than the sp hybrid orbitals).

Figure: sp2 Hybrid Orbitals. Hybridization of the 2s and two of the 2p AOs forms three sp2 hybrid orbitals, oriented 120° with respect to each other in the same plane; one of the 2p AOs remain unhybridized (move the slider around to see the before/after of hybridization).

sp3 Hybridization

Combining one valence s AO and all three valence p AOs produces four degenerate sp3 hybridized orbitals, as shown in Figure: sp3 Hybrid Orbitals (second figure below) for the case of 2s and 2p AOs. The four sp3 hybridized orbitals are oriented at 109.5° with respect to each other, each pointing toward a different corner of a tetrahedron—a tetrahedral geometry.

A tetrahedron is a three-dimensional object that has four equilateral triangular faces and four apexes (corners). All four corners are equivalent. See Figure: Tetrahedron.

Figure: Tetrahedron. A tetrahedron has four equilateral triangular sides and four apexes (corners). Three views of a tetrahedron are shown at left. Four sp3 hybridized orbitals point from the tetrahedron’s center toward the four corners. These hybrid orbitals form four bonds that point toward each corner of a tetrahedron. The angle between any two bonds is 109.5 degrees.

An sp3 hybrid orbital has three times as much “p” character as “s” character, hence the superscript “3” in its name. The sp3 hybrid orbitals are higher in energy than the sp2 hybrid orbitals, as illustrated in Figure: sp3 Hybrid Orbitals.

Figure: sp3 Hybrid Orbitals. Hybridization of the 2s and all three 2p AOs forms four sp3 hybrid orbitals, oriented 109.5° with respect to each other (move the slider around to see the before/after of hybridization).
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Chemistry 109 Fall 2021 by John Moore, Jia Zhou, and Etienne Garand is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.