Orbital hybridisation

Orbital hybridisation is a concept in chemistry that describes the combination of atomic orbitals into new hybrid orbitals (with different energies, shapes, etc., than the component atomic orbitals) suitable for the pairing of electrons to form chemical bonds in molecules. The hybrid orbitals are very useful in the explanation of the shape of molecular orbitals for molecules. It is an integral part of the valence bond theory.

Overview

Orbital hybridisation involves the mixing of two or more atomic orbitals to form the same number of hybrid orbitals, each having a different shape and energy than the original atomic orbitals. Hybrid orbitals are very effective in explaining the geometry of molecules. The type of hybridisation depends on how many and which types of atomic orbitals are involved.

Types of Hybridisation

There are several types of hybridisation that can occur, depending on the number and types of orbitals mixed:

• sp Hybridisation: This occurs when one s orbital mixes with one p orbital, resulting in two sp hybrid orbitals. Molecules with a linear geometry, such as BeCl2, exhibit sp hybridisation.
• sp^2 Hybridisation: In sp^2 hybridisation, one s orbital mixes with two p orbitals, forming three sp^2 hybrid orbitals. This hybridisation is observed in molecules with a trigonal planar geometry, such as BF3.
• sp^3 Hybridisation: This involves the mixing of one s orbital with three p orbitals to form four sp^3 hybrid orbitals, as seen in molecules with a tetrahedral geometry, like CH4.
• sp^3d Hybridisation: In this type, one s orbital, three p orbitals, and one d orbital mix together to form five sp^3d hybrid orbitals, suitable for trigonal bipyramidal geometries, as in PCl5.
• sp^3d^2 Hybridisation: This hybridisation involves one s orbital, three p orbitals, and two d orbitals mixing to form six sp^3d^2 hybrid orbitals, seen in octahedral geometries, like SF6.

Mechanism

The process of hybridisation involves the reshuffling of the valence electrons of an atom. This reshuffling allows the atom to release or absorb energy and thereby facilitates the formation of hybrid orbitals with new orientations and energy levels. These new hybrid orbitals are then available to form bonds with other atoms.

Importance in Molecular Geometry

The concept of orbital hybridisation is crucial in explaining the geometry and bonding properties of molecules. It helps in understanding why certain molecules have specific shapes and how the electrons are distributed among the orbitals in a molecule. For example, the tetrahedral shape of methane (CH4) is explained by the sp^3 hybridisation of the carbon atom's orbitals.

Limitations

While orbital hybridisation provides a useful model for understanding molecular geometry and bonding, it has its limitations. It is primarily applicable to molecules with localized bonds and may not accurately describe the bonding in molecules with delocalized electrons, such as those involved in conjugated systems or aromatic compounds.

See Also

• Molecular orbital theory
• Valence bond theory
• Chemical bond
• Molecular geometry