Molecular Geometry, VSEPR and Polarity

GENERAL CHEMISTRY / 3D MOLECULES

VSEPR, Geometry & Molecular Polarity

Use central-atom electron domains and lone pairs to predict 3D shapes and bond-dipole cancellation.

Learning objectives

Count central-atom electron domains correctly.
Distinguish electron geometry from molecular geometry using AXE notation.
Recognize when symmetric bonds cancel to yield nonpolar molecules.

1. Count regions, not bond lines

Each bond to a neighboring atom counts as one electron domain around the central atom, even if it is a double or triple bond. Each lone pair counts as an additional region. CO₂ therefore has two domains around C.

Worked example

CO₂: two C=O double bonds → AX₂ → linear (180°).

2. Choose electron-domain geometry

Two, three, four, five, or six domains correspond ideally to linear, trigonal planar, tetrahedral, trigonal bipyramidal, and octahedral electron-domain geometries, respectively.

Worked example

NH₃: four domains → tetrahedral electron geometry.

3. Remove lone pairs from the shape name

Molecular geometry describes the positions of bonded atoms, not the lone pairs. In NH₃ (AX₃E), the molecular geometry is trigonal pyramidal. H₂O (AX₂E₂) is bent.

Worked example

H₂O: two bonding domains + two lone pairs → bent, about 104.5°.

4. Use angles as guides, not absolute rules

Ideal bond angles are 180°, 120°, 109.5°, 90°/120°, or 90° as appropriate. Lone pairs and chemical differences commonly distort real bond angles. Some compounds (such as solid PCl₅) have different structures in different phases.

Worked example

SF₄: AX₄E → seesaw; deviations from ideal trigonal-bipyramidal angles.

5. Evaluate molecular polarity

First identify whether bonds are polar. Then consider their vector arrangement. A symmetric molecule with identical outer atoms may have cancelling dipoles. CO₂ is nonpolar, while bent H₂O is polar.

Worked example

XeF₄: square planar with two opposite lone pairs; Xe–F dipoles cancel in the ideal geometry.

6. Rotate the interactive models

Use the Molecular Structure Viewer to compare ball-and-stick arrangements. These are simplified instructional models and do not replace measured experimental geometries.

Worked example

Compare CH₄ (tetrahedral) and NH₃ (trigonal pyramidal).

Common mistakes

Treating a double bond as two electron domains.
Counting terminal-atom lone pairs when naming the central-atom geometry.
Assuming every molecule with polar bonds has a net dipole.
Treating idealized VSEPR angles as exact measurements.

Check your understanding

Why are CO₂ and H₂O different in molecular polarity?

Show explanation

CO₂ is linear and symmetric, so the C=O dipoles cancel. H₂O is bent, so the O–H dipoles have a net resultant.