Lesson 8 of 12 · 8 min
Hybridisation
NCERT § "Hybridisation"
The gas in a city kitchen pipeline is mostly methane, CH₄. Carbon has only two unpaired electrons in its ground state, and its p orbitals are at 90°. Yet methane has four identical bonds at 109.5°.
The lesson in notes
In short
Hybridisation is the mixing of atomic orbitals of similar energy on the same atom to form an equal number of equivalent hybrid orbitals.
Only orbitals of similar energy hybridise; hybrid orbitals form σ bonds or hold lone pairs, never π bonds.
sp: one s + one p, linear, 180°, 50% s character (BeCl₂; each carbon in ethyne).
sp²: one s + two p, trigonal planar, 120°, 33% s character (BCl₃; each carbon in ethene).
sp³: one s + three p, tetrahedral, 109.5° (109°28′), 25% s character (CH₄); NH₃ and H₂O are also sp³, with angles squeezed by lone pairs.
In ethene each carbon uses sp² orbitals for σ bonds and the unhybridised p orbital for the π bond; in ethyne each carbon forms 2 σ bonds and 2 π bonds.
sp³d (dz² orbital used) gives a trigonal bipyramid, as in PCl₅; the two axial P–Cl bonds are longer and weaker than the three equatorial bonds, which makes PCl₅ reactive.
sp³d² (dz² and dx²−y² used) gives an octahedron, as in SF₆, with six equal S–F bonds at 90°.
Quick rule: count σ bonds plus lone pairs on the central atom; 2, 3, 4, 5 and 6 correspond to sp, sp², sp³, sp³d and sp³d².