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What is hybridisation in chemistry?

Class 11 ChemistryNCERT chapter: Chemical Bonding and Molecular Structureइसे हिंदी में पढ़ें

Exam-ready answer

Hybridisation is the process of intermixing of atomic orbitals of slightly different energies so as to redistribute their energies, resulting in a new set of orbitals of equivalent energy and shape, called hybrid orbitals. The number of hybrid orbitals equals the number of atomic orbitals mixed. For example, in methane carbon's one 2s and three 2p orbitals form four sp³ hybrid orbitals.

Explanation

Valence bond theory could not explain why the four C–H bonds in methane are identical and point to the corners of a tetrahedron, since carbon's 2s and 2p orbitals differ in energy and shape. Pauling explained this with hybridisation. In the excited state, carbon (2s¹2p³) mixes its one 2s and three 2p orbitals to form four equivalent sp³ hybrid orbitals, each with 25% s and 75% p character.

Hybrid orbitals point in definite directions and keep as far apart as possible, which fixes the molecule's shape. They also overlap better than pure orbitals, so they form stronger bonds. Only valence-shell orbitals of nearly equal energy hybridise; promotion of an electron is not essential, and filled orbitals can also take part. Hybridisation applies to the central atom, not to the molecule as a whole.

Three common types: sp, one s with one p, gives two orbitals at 180°, linear, as in BeCl₂. sp², one s with two p, gives three orbitals at 120°, trigonal planar, as in BCl₃. sp³, one s with three p, gives four orbitals at 109°28′, tetrahedral, as in CH₄. In NH₃ and H₂O nitrogen and oxygen are also sp³, but lone pairs squeeze the bond angles to about 107° and 104.5°.

Points to remember

  1. Hybridisation: intermixing of atomic orbitals of slightly different energies to give an equal number of equivalent hybrid orbitals.
  2. Number of hybrid orbitals formed = number of atomic orbitals that hybridise.
  3. sp: linear, 180° (BeCl₂); sp²: trigonal planar, 120° (BCl₃); sp³: tetrahedral, 109°28′ (CH₄).
  4. Only valence-shell orbitals of nearly equal energy hybridise; promotion of electrons is not essential.
  5. Hybrid orbitals form stronger bonds than pure atomic orbitals because they overlap more effectively.

Exam tip

Give the definition, then show CH₄ step by step: ground state, excited state, mixing of 2s and three 2p into four sp³ orbitals. Draw the tetrahedral shape with the 109°28′ angle and add a small table of sp, sp², sp³ with shape, angle and example.

Questions students also ask

What is the hybridisation of carbon in ethene and ethyne?
In ethene (C₂H₄), each carbon is sp² hybridised; the three sp² orbitals form σ bonds at 120°, and the unhybridised p orbitals overlap sideways to form one π bond. In ethyne (C₂H₂), each carbon is sp hybridised, giving a linear molecule with one σ and two π bonds between the carbons.
Why is the bond angle in water less than 109°28′?
Oxygen in water is sp³ hybridised, with two hybrid orbitals forming O–H bonds and two holding lone pairs. Lone pair–lone pair repulsion is stronger than bond pair–bond pair repulsion, so the O–H bonds are pushed closer and the angle reduces to about 104.5°.
Is hybridisation a real physical process?
Hybridisation is a model used to explain the observed shapes and equal bond lengths of molecules. Atoms do not first hybridise and then bond in separate steps. It is a convenient way of describing the orbitals of the central atom in a molecule whose geometry is already known.

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