# 9.20: Hybrid Orbitals - sp and sp²

Romeo and Juliet were two of the greatest-known fictional lovers of all time. Their embrace allowed no other person to be a part of it—they only wanted to be with each other. It took outside intervention to get them away from one another. Paired electrons are similar to Romeo and Juliet. They do not bond covalently until they are unpaired; then, they can become a part of a larger chemical structure.

### sp Hybridization

A beryllium hydride $$\left( \ce{BeH_2} \right)$$ molecule is predicted to be linear by VSEPR. The beryllium atom contains all paired electrons and so must also undergo hybridization. One of the $$2s$$ electrons is first promoted to the empty $$2p_x$$ orbital (see figure below).

Now the hybridization takes place only with occupied orbitals, and the result is a pair of $$sp$$ hybrid orbitals. The two remaining $$p$$ orbitals ($$p_y$$ and $$p_z$$) do not hybridize and remain unoccupied (see figure below).

The geometry of the $$sp$$ hybrid orbitals is linear, with the lobes of the orbitals pointing in opposite directions along one axis, arbitrarily defined as the $$x$$-axis (see figure below). Each can bond with a $$1s$$ orbital from a hydrogen atom to form the linear $$\ce{BeH_2}$$ molecule.

Other molecules whose electron domain geometry is linear and for whom hybridization is necessary also form $$sp$$ hybrid orbitals. Examples include $$\ce{CO_2}$$ and $$\ce{C_2H_2}$$, which will be discussed in further detail later.

### sp² Hybridization

Boron trifluoride $$\left( \ce{BF_3} \right)$$ is predicted to have a trigonal planar geometry by VSEPR. First, a paired $$2s$$ electron is promoted to the empty $$2p_y$$ orbital (see figure below).

This is followed by hybridization of the three occupied orbitals to form a set of three $$sp^2$$ hybrids, leaving the $$2p_z$$ orbital unhybridized (see figure below).

The geometry of the $$sp^2$$ hybrid orbitals is trigonal planar, with the lobes of the orbitals pointing towards the corners of a triangle (see figure below). The angle between any two of the hybrid orbital lobes is $$120^\text{o}$$. Each can bond with a $$2p$$ orbital from a fluorine atom to form the trigonal planar $$\ce{BF_3}$$ molecule.

Other molecules with a trigonal planar electron domain geometry form $$sp^2$$ hybrid orbitals. Ozone $$\left( \ce{O_3} \right)$$ is an example of a molecule whose electron domain geometry is trigonal planar, though the presence of a lone pair on the central oxygen makes the molecular geometry bent. The hybridization of the central $$\ce{O}$$ atom of ozone is $$sp^2$$.

## Summary

• Paired electrons can be hybridized and then participate in covalent bonding.