2.7 Hybridization

Hybridization (sp/sp²/sp³)

Rotate 10 real molecules and watch σ and π orbitals overlap side by side. Count σ bonds plus lone pairs to get the steric number, then read off sp, sp², or sp³ and the ideal angle.

sp / sp² / sp³σ vs π OverlapSteric Number10 Molecules
Topic 2.7

VSEPR and Hybridization

Based on the relationship between Lewis diagrams, VSEPR theory, bond orders, and bond polarities: (i) explain structural properties of molecules; (ii) explain electron properties of molecules.

VSEPR theory uses Coulombic repulsion between electron domains as the basis for predicting how electron pairs arrange around a central atom. Domains — bonding pairs (a multiple bond counts as one domain) and lone pairs — get as far apart as geometry allows.

EK 2.7.A.2 lists exactly what you must be able to predict from a Lewis diagram plus VSEPR:

  • Molecular geometry: linear, trigonal planar, tetrahedral, trigonal pyramidal, bent, trigonal bipyramidal, seesaw, T-shaped, octahedral, square pyramidal, square planar.
  • Bond angles.
  • Relative bond energies based on bond order.
  • Relative bond lengths (multiple bonds and atomic radius effects).
  • Presence of a dipole moment.
  • Hybridization of valence orbitals.

The method: count electron domains on the central atom → that gives the electron-domain geometry and the ideal angle → subtract lone pairs → that gives the molecular shape. Lone pairs occupy more space than bonding pairs, so each one compresses the remaining angles below the ideal value. NH₃ (4 domains, 1 lone pair) has angles of about 107°, and H₂O (4 domains, 2 lone pairs) about 104.5°, both under the tetrahedral 109.5°.

Hybridization is named directly from domain count: 2 domains → sp, ideal angle 180°; 3 domains → sp², 120°; 4 domains → sp³, 109.5°. That is the entire required mapping.

Sigma and pi bonds. Bonds form from overlap of atomic orbitals. End-on overlap gives a sigma (σ) bond; side-on overlap of p orbitals gives a pi (π) bond. Sigma overlap is stronger, so sigma bonds have greater bond energy than pi bonds. Counting is mechanical:

  • Single bond = 1 σ · Double bond = 1 σ + 1 π · Triple bond = 1 σ + 2 π

A pi bond also prevents rotation about the bond axis, which is what makes geometric (cis/trans) isomers possible.

Polarity. A molecule has a net dipole if the individual bond dipoles do not cancel by symmetry. CO₂ is linear with two identical polar bonds pointing opposite ways — nonpolar. H₂O is bent, so its two O–H dipoles add — polar. Symmetric shapes with identical terminal atoms (linear, trigonal planar, tetrahedral) cancel; asymmetric shapes (bent, trigonal pyramidal) do not.

Key points

  • Count domains → electron geometry and ideal angle. Subtract lone pairs → molecular shape.
  • Domains map directly to hybridization: 2 = sp, 3 = sp², 4 = sp³.
  • Every bond has exactly one sigma; extra bonds in a multiple bond are pi.
  • Polarity = bond polarity + geometry. Both are required; neither alone decides.

Common mistakes

  • A double bond is one domain, not two. CO₂ has two domains around carbon and is linear.
  • Electron geometry vs. molecular geometry. H₂O has tetrahedral electron geometry but bent molecular geometry. Read which the question asks for.
  • Lone pairs count for hybridization. Water is sp³ even though only two of the four domains are bonds.
  • Symmetric molecules with polar bonds can be nonpolar. CCl₄ and CO₂ are the standard examples.
  • Angles are approximate. Say "slightly less than 109.5°", not exactly 107° unless the value is given.

Worked example

For SO₂: draw the Lewis structure, determine the electron-domain geometry, molecular geometry, approximate bond angle, hybridization of sulfur, number of sigma and pi bonds, and whether the molecule is polar.

Lewis: 6 + 2(6) = 18 valence electrons. Sulfur is central. One S=O double bond, one S–O single bond, and one lone pair on sulfur (two equivalent resonance structures).

Electron domains on S: 2 bonding domains + 1 lone pair = 3 domains.

Electron-domain geometry: trigonal planar (ideal 120°).

Molecular geometry: bent (3 domains, 1 lone pair).

Bond angle: slightly less than 120° — the lone pair repels more strongly than the bonding pairs. (Measured: about 119°.)

Hybridization: 3 domains → sp².

Sigma and pi: two S–O connections → 2 sigma bonds; the double bond adds 1 pi → 2 σ, 1 π.

Polarity: polar. The two S–O bond dipoles point toward oxygen, and because the molecule is bent they do not cancel — they add to a net dipole pointing away from the sulfur lone pair. (Contrast CO₂, which is linear and therefore nonpolar.)

Bond lengths: because of resonance, both S–O bonds are identical, with a bond order of 1.5.

Full notes for topic 2.7 →