Upper secondary · 12 min

Predicting molecular shapes with VSEPR

Step through the five basic AXₙ geometries and see how electron-pair repulsion fixes the bond angles.

Goal

Connect each electron-pair count to a geometry and a bond angle, and name a real molecule for each.

Apparatus and reagents

Molecular model kit or the simulation below; a table of AXₙEₘ classes.

Procedure

Select AX₂ and note the angle between the two ligands.
Move through AX₃ and AX₄ and compare 120° with 109.5°.
For AX₅, distinguish the two axial ligands from the three equatorial ones.
Finish with AX₆ and check that every angle is 90° or 180°.

What to observe

Each additional electron pair pushes the ligands onto a new optimum: 180° for two, 120° in a plane for three, 109.5° in space for four.
In the trigonal bipyramid the axial and equatorial positions are not equivalent — they see different angles (90° vs 120°) to their neighbours.

Explanation

VSEPR treats each bonding (and non-bonding) electron pair around the central atom as a region of negative charge; the pairs arrange to maximise their mutual distances. The AXₙEₘ notation counts n bonded atoms X and m lone pairs E; this lab shows the m = 0 series. Lone pairs repel more strongly than bonding pairs, so AX₃E (NH₃, 107°) and AX₂E₂ (H₂O, 104.5°) are squeezed below the ideal 109.5°.

History of the experiment

Van ’t Hoff proposed the tetrahedral carbon in 1874 to explain optical isomerism, long before quantum mechanics. Sidgwick and Powell systematised electron-pair arrangements in 1940, and Gillespie and Nyholm turned the idea into the VSEPR rules taught today in 1957.

Chemists behind it

Gilbert N. Lewis Jacobus Henricus van ’t Hoff