Hybridization

Atoms can form bonds with each other by sharing unpaired electrons such that
each bond contains two electrons. In Topic A1, we identified that a carbon atom
has two unpaired electrons and so we would expect carbon to form two bonds.
However, carbon forms four bonds! How does a carbon atom form four bonds
with only two unpaired electrons?
So far, we have described the electronic configuration of an isolated carbon
atom. However, when a carbon atom forms bonds and is part of a molecular structure,
it can emixf the s and p orbitals of its second shell (the valence shell). This is
known as hybridization and it allows carbon to form the four bonds which we
observe in reality.
There are three ways in which this mixing process can take place.
œ the 2s orbital ismixedwith all three 2p orbitals. This isknownas sp3 hybridization;
œ the 2s orbital is mixed with two of the 2p orbitals. This is known as sp2
hybridization;
œ the 2s orbital is mixed with one of the 2p orbitals. This is known as sp
hybridization.

In sp3 hybridization, the s and the p orbitals of the second shell are ‘mixed’
to form four hybridized sp3 orbitals of equal energy.
Each hybridized orbital contains a single unpaired electron and so four
bonds are possible.
Each sp3 orbital is shaped like a deformed dumbbell with one lobe much
larger than the other. The hybridized orbitals arrange themselves as far
apart from each other as possible such that the major lobes point to the corners
of a tetrahedron. sp3 Hybridization explains the tetrahedral carbon in
saturated hydrocarbon structures.
Sigma (σ) bonds are strong bonds formed between two sp3 hybridized carbons
or between an sp3 hybridized carbon and a hydrogen atom. A σ bond
formed between two sp3 hybridized carbon atoms involves the overlap of
half filled sp3 hybridized orbitals from each carbon atom. A σ bond formed
between an sp3 hybridized carbon and a hydrogen atom involves a halffilled
sp3 orbital from carbon and a half-filled 1s orbital from hydrogen.
Nitrogen, oxygen, and chlorine atoms can also be sp3 hybridized in organic
molecules. This means that nitrogen has three half-filled sp3 orbitals and can
form three bonds which are pyramidal in shape. Oxygen has two half-filled
sp3 orbitals and can form two bonds which are angled with respect to each
other. Chlorine has a single half-filled sp3 orbital and can only form a single
bond. All the bonds which are formed are σ bonds.
adapted from G. L. Patrick
Department of Chemistry and Chemical Engineering,
Paisley University, Paisley, Scotland