Topic 14.2: Hybridization

14.2.1 Describe σ and π bonds 

When two atomic orbitals overlaps along the bond axis - an imaginary line between the two nuclei - the bond is described as a sigma bond. Denoted as σ. This type of bond forms by the overlaps of s orbitals, p orbitals and hybrid orbitals. 

When two p orbitals overlaps sideways, the electron density of the molecular orbital is concentrated in two regions, above and below the plane of the bond angle. This type of bond only forms by the overlap of p orbitals alongside the formation of a sigma bond. In other words, pi bonds only form within double bond or triple bond.

14.2.2 Explain hybridization in terms of the mixing of atomic orbitals to form new orbitals for bonding.


Carbon forms four covalent bonds. Yet if we consider the electron configuration in the carbon atom, we would not predict this as it has only two singly occupied orbitals available for bonding.

Because carbon can have 4 covalent bonds, this indicates that this lowest energy or ground state electron configuration changes during bonding. The electron in 2s is promoted to 2p by the process of excitation. The atoms now have 4 singly occupied orbitals available for bonding.

14.2.3 Identify and explain the relationships between Lewis structures, molecular shapes and types of hybridization (sp, sp^2 and sp^3)


Though this process can explain the four covalent bonds, the three p orbitals should have a slightly higher energy than the s orbital. So if there were used in bonding, we would expect unequal bonds. However methane has four identical bonds, so we can safely assume that the orbitals have been made equal during the bonding process. They form new hybrid atomic orbitals which are the same as each other, but different from the original orbitals.This mixing of orbitals is called hybridization.

sp^3

When carbon forms four single bonds, it under goes sp^3 hybridization, producing 4 equal orbitals. Note that it should have a tetrahedral arrangement.

sp^2

When carbon forms a double bond, it undergoes sp^2 hybridization, producing three equal orbtials. This typically forms, double bonds as the remaining p orbital becomes part of the pi bond. Note that it should have a planar triangular arrangement.

sp

When carbon forms a triple bond, it under goes sp hybridization, producing two equal orbitals. Note that it should have a linear arrangement.

These coalesce into a cylinder of negative charge around the atom, making the molecule susceptible to attack by electrophilic reagents.

Non-bonding pairs can also take part in hybridization. The non-bonding pair in ammonia, NH3, resides in a sp^3 orbital.

Note: Look at the number of charge centres, then the shape of the molecule. Then you would be able to find the type of hybridization.