Upper secondary · 15 min

Molecular geometry in 3D: electron pairs rule

Turn lone pairs on and off for every AXₙEₘ skeleton — from linear $\ce{CO2}$ to octahedral $\ce{SF6}$ — and watch bond angles shrink.

Goal

Link each shape number to a geometry name, an ideal angle, and a real molecule; explain why lone pairs compress the angles.

Apparatus and reagents

The 3D viewer below; a printed table of the ten VSEPR classes AX₂ to AX₆ with E variants.

Procedure

With lone pairs hidden, sweep shapes 0→3 (linear, trigonal planar, tetrahedral) and record the angles 180°, 120°, 109.5°.
Continue through shapes 4–6 (trigonal bipyramid, seesaw, square planar family) and spot the axial vs equatorial sites.
Now toggle lone pairs ON at every step: compare AX₂E₂ ( $\ce{H2O}$ , bent) with AX₂ ( $\ce{CO2}$ , linear) and AX₃E ( $\ce{NH3}$ , pyramidal) with AX₃ ( $\ce{BF3}$ , planar).
Pick any three real molecules and predict their shape number and angle before checking with the viewer.

What to observe

With lone pairs shown, bond angles fall below the ideal values: 107° for $\ce{NH3}$ , 104.5° for $\ce{H2O}$ , less than 90° for $\ce{SF4}$ -like seesaws.
Adding a pair to the shell always moves the ligands to a new maximum-separation geometry — the electron-pair count, not the atom count, fixes the shape.

Explanation

VSEPR counts electron-pair domains around the central atom; pairs repel and settle at maximum separation. Lone pairs occupy more angular space than bonding pairs (they belong to one nucleus only), so each E squeezes X–A–X angles by ~2–3°. That is why $\ce{H2O}$ (104.5°) is tighter than $\ce{NH3}$ (107°), itself tighter than $\ce{CH4}$ (109.5°).

Chemists behind it

Gilbert N. Lewis Jacobus Henricus van ’t Hoff