sp² Hybridization

The valence orbitals of a central atom surrounded by three regions of electron density consist of a set of three sp2 hybrid orbitals and one unhybridized p orbital. These orbitals form though sp2 hybridization. It involves mixes of one s orbital and two p orbitals, forming a trigonal planar geometry. Rsulting in the formation of three identical hybrid orbitals

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Quantum mechanics produces the “plump” orbital lobes. For clarity, we sometimes depict these orbitals in a thinner form and exclude the monor lobes, shown below. These “thinner” representations simplify illustrations by preventing the overcrowding of features. We will adopt this approach whenever the true view becomes too complex to visualize easily.

This alternate way of drawing the trigonal planar sp2 hybrid orbitals is sometimes used in more crowded figures.

Borane, BH3, molecule

The observed structure of the borane molecule, BH3, suggests sp2 hybridization for boron in this compound. The molecule adopts a trigonal planar shape, with the boron atom forming three bonds to hydrogen atoms (Figure 2, below). We can compare orbitals and electron distribution in an isolated boron atom in BH3. This comparison is illustrate in the oribtal energy level diagram in Figure 3. The three valence electrons of boron redistribute into three sp2 hybrid orbitals. Note that each boron electron pairs with a hydrogen electron during the formation of B-H bonds.

BH3 is an electron-deficient molecule with a trigonal planar structure.
In an isolated B atom, there are one 2s and three 2p valence orbitals. When boron is in a molecule with three regions of electron density, three of the orbitals hybridize and create a set of three sp2 orbitals and one unhybridized 2p orbital. The three half-filled hybrid orbitals each overlap with an orbital from a hydrogen atom to form three σ bonds in BH3.

Any central atom surrounded by three regions of electron density will exhibit sp2 hybridization. This includes molecules with a lone pair on the central atom, such as ClNO, or molecules with two single bonds and a double bond connected to the central atom, as in formaldehyde, CH2O, and ethene, H2CCH2.

The central atom(s) in each of the structures shown contain three regions of electron density and are sp2 hybridized. As we know from the discussion of VSEPR theory, a region of electron density contains all of the electrons that point in one direction. A lone pair, an unpaired electron, a single bond, or a multiple bond would each count as one region of electron density.