Bonding
Hybridization
Mixing atomic orbitals to explain molecular geometry — sp, sp², sp³
Hybridization is the mixing of atomic orbitals (s, p, d) to form new hybrid orbitals that better explain molecular geometry. Pure s and p orbitals can't account for tetrahedral CH₄ angles. Solution: 1 s + 3 p orbitals → 4 sp³ hybrid orbitals (109.5° apart). Variations: sp (linear, 180°), sp² (trigonal planar, 120°), sp³ (tetrahedral), sp³d (trigonal bipyramidal), sp³d² (octahedral). Concept by Linus Pauling (1931). Hybridization explains: bond angles, double/triple bonds (σ + π), restricted rotation. Foundation of organic chemistry.
- sp hybridization2 hybrids (linear, 180°); e.g., BeCl₂, CO₂
- sp² hybridization3 hybrids (trigonal planar, 120°); e.g., BF₃, ethene
- sp³ hybridization4 hybrids (tetrahedral, 109.5°); e.g., CH₄, NH₃, H₂O
- sp³d5 hybrids (trigonal bipyramidal); e.g., PCl₅
- sp³d²6 hybrids (octahedral); e.g., SF₆
- Concept byLinus Pauling, 1931
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Why hybridization matters
- Geometry prediction. Explains observed bond angles.
- Multiple bonds. σ + π framework.
- Organic chemistry. Carbon's amazing versatility.
- Spectroscopy. Hybridization affects IR, NMR.
- Reactivity. Different hybrids = different reactivity.
- Bond strength. Triple > double > single.
- Molecular planarity. Restriction in π systems.
Common misconceptions
- Hybridization is real physical process. Mathematical framework; orbitals are descriptions.
- Hybrids are physical objects. Linear combinations of atomic orbitals.
- sp³ always has 109.5° angles. Lone pairs distort.
- Hybridization energy is always favorable. Net energy includes bond formation.
- All atoms hybridize. Not for atoms with single bonds and inert pairs.
- d-orbital hybridization is well-established. Modern view: less involved.
Frequently asked questions
Why is hybridization needed?
Pure atomic orbitals don't predict observed geometry. Carbon ground state: 2s² 2p². Two unpaired electrons → expect divalency. But carbon forms 4 bonds (CH₄). And CH₄ has 4 equivalent bonds at 109.5°, not 90° (pure p orbital angle). Mixing 2s + 3 2p orbitals gives 4 equivalent sp³ hybrids — 4 bonds at 109.5°.
How do you determine hybridization?
Count electron domains (bonds + lone pairs) around atom. 2 domains: sp. 3: sp². 4: sp³. 5: sp³d. 6: sp³d². Same as VSEPR: hybridization matches electron geometry. Each domain occupies one hybrid orbital. Examples: CH₄ → sp³. NH₃ → sp³ (N has 4 domains: 3 bonds + 1 lone pair). C in C₂H₂ → sp.
What's a sigma bond?
Bond formed by direct (head-on) overlap of orbitals. Single bonds are σ. Atoms can rotate around σ bonds. All bonds in CH₄ are σ. In double bond: 1 σ + 1 π. In triple bond: 1 σ + 2 π.
What's a pi bond?
Bond formed by side-by-side overlap of unhybridized p orbitals. Found in double and triple bonds. π bonds prevent rotation (would break π overlap). Double bond C=C: σ from sp² overlap + π from p-p overlap. Triple bond C≡C: σ + 2 π. π bonds weaker than σ but provide rigidity to molecules.
How does hybridization explain ethene (C₂H₄)?
Each C: sp² hybridized. Three sp² orbitals (in plane) form bonds with H, H, and other C (σ bonds). One unhybridized p-orbital (perpendicular to plane) on each C. Two p-orbitals overlap side-by-side → π bond. Total: 1 σ (C-C) + 1 π (C=C) = double bond. Planar geometry; rotation restricted by π bond.
What about d-orbitals?
Period 3+ atoms can use d-orbitals in hybridization. PCl₅: phosphorus uses 5 hybrids (1 s + 3 p + 1 d) = sp³d. SF₆: 6 hybrids (sp³d²). This explains expanded octets. However, modern computational studies suggest d-orbital involvement is small — alternative theories (3-center 4-electron bonds) explain better. Still useful for predicting geometry.
What's the difference between hybridization and orbital mixing?
Hybridization specifically refers to mixing orbitals on the same atom to prepare for bonding. Different from molecular orbital theory (MO) where atomic orbitals on different atoms combine to form bonding/antibonding MOs. Hybridization is part of valence bond theory; MO theory is alternative bonding model. Both have merits; chemists use hybridization for organic molecules; MO for small inorganics.