http://www.ck12.org Chapter 9. Covalent Bonding
Valence Bond Theory
Covalent bonds form when the electron clouds of two atoms overlap with each other. In a simple H 2 molecule, the
single electron in each atom becomes attracted to the nucleus of the other atom in the molecule as the atoms come
closer together. Other covalent bonds form in the same way as unpaired electrons from two atoms “match up” to
form the bond. In a fluorine atom, there is an unpaired electron in one of the 2p orbitals. When a F 2 molecule forms,
the 2p orbitals from each of the two atoms overlap to produce the F−F covalent bond. The overlapping orbitals do
not have to be of the same type to form a covalent bond. For example, in a molecule of HF, the 1s orbital of the
hydrogen atom overlaps with the 2p orbital of the fluorine atom (Figure9.22):
FIGURE 9.22
In a molecule of hydrogen fluoride (HF),
the covalent bond occurs due to an over-
lap between the 1s orbital of the hydrogen
atom and the 2p orbital of the fluorine
atom.In essence, any covalent bond results from a combination of atomic orbitals. This idea forms the basis for a quantum
mechanical theory called valence bond (VB) theory.Valence bond theorystates that covalent bonds are formed by
the overlap of partially filled atomic orbitals.
Hybrid Orbitals
The bonding scheme described by valence bond theory must account for molecular geometries as predicted by
VSEPR theory. To do that, we must introduce a concept called hybrid orbitals.
sp^3 Hybridization
Unfortunately, overlap of existing atomic orbitals (s, p, etc.) is not sufficient to explain some of the bonding and
molecular geometries that are observed. Consider the carbon atom in the methane (CH 4 ) molecule. An isolated
carbon atom has an electron configuration of 1s^2 2s^2 2p^2 , meaning that it has two unpaired electrons in its 2p orbitals,
as shown below.