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2.2.4—VSEPR model

Syllabus
First assessment 2025
Objective
2.2.4
Level
HL

VSEPR Geometry

VSEPR predicts molecular shape by arranging electron domains around a central atom to minimize repulsion. Lone pairs repel more strongly than bonding pairs and therefore alter common bond angles.

BeCl2 is Cl-Be-Cl with single bonds and three lone pairs on each chlorine; CO2 is O=C=O with two lone pairs on each oxygen; C2H2 is H-C≡C-H; all three central angles are 180 degrees.
BF3 has three bonding domains, no central lone pair, and 120-degree trigonal-planar geometry; NO2-minus has two bonding domains and one central lone pair; nitrite is bracketed with overall negative charge and identified as one resonance form; nitrite bond angle is less than 120 degrees.
CH4 has four bonds, zero lone pairs, tetrahedral geometry, and 109.5 degrees; NH3 has three bonds, one lone pair, trigonal-pyramidal geometry, and 107 degrees; H2O has two bonds, two lone pairs, bent geometry, and 104.5 degrees; wedge/dash structures agree with the ball-and-stick models.
Step Decision
1 Count bonding and lone-pair electron domains
2 Assign electron-domain geometry
3 Ignore lone pairs when naming molecular geometry
4 Adjust expected bond angles for lone-pair repulsion

NH₃ has four electron domains around N, so its electron-domain geometry is tetrahedral but its molecular shape is trigonal pyramidal; the lone pair compresses the H–N–H angle below 109.5°. Count a double bond as one domain and distinguish electron geometry from the shape named using atoms only.

Applying VSEPR

[ 2 ]

Deduce the electron domain geometry and molecular geometry of SO2\mathrm{SO}_{2}.

Electron domain geometry:

Molecular domain geometry:

The Covalent Model Summary

Retrieve the covalent pathway: shared pairs and bond order lead to geometry, polarity and molecular polarity; structure determines network properties, IMF behaviour and chromatography; HL representations extend to resonance, formal charge, sigma/pi bonds and hybridization.

Check the representation first, then count domains, apply geometry, identify polarity or forces, and connect the structure to the requested property or HL bonding description.