Electron pairs around a central atom push each other as far apart as they can. Count the pairs, remember that lone pairs push harder, and you can predict the shape and bond angles of almost any small molecule or ion.
In VSEPR theory the electron pairs around a central atom repel one another and arrange themselves as far apart as possible, which fixes the shape. Count the bonding pairs (a double or triple bond counts as one region) and the lone pairs. Lone pairs are held closer to the nucleus and repel more than bonding pairs: lone pair–lone pair > lone pair–bond pair > bond pair–bond pair. Each lone pair closes the remaining bond angles by roughly 2.5°, so CH4 is 109.5°, NH3 107° and H2O 104.5°. The shape is named from the positions of the atoms, not the lone pairs.
Load each molecule in the 3D simulator and rotate it. Start with the series CH4, NH3, H2O and watch the bond angle shrink as bonding pairs are replaced by lone pairs.
Apply the four steps to ions and molecules with double bonds. A double bond counts as a single region of electron density when you decide the shape.
| Species | Bonding regions | Lone pairs | Shape | Angle |
|---|---|---|---|---|
| NH4+ | 4 | 0 | tetrahedral | 109.5° |
| H3O+ | 3 | 1 | pyramidal | 107° |
| NH2− | 2 | 2 | non-linear | 104.5° |
| CO2 | 2 (double bonds) | 0 | linear | 180° |
| SF6 | 6 | 0 | octahedral | 90° |
Four quick checks. Each correct answer earns XP and records this skill on your profile.
What is the shape of a methane molecule, CH4?
What is the bond angle in boron trifluoride, BF3?
Why is the H–N–H angle in NH3 smaller than the H–C–H angle in CH4?
What is the shape of a sulfur hexafluoride molecule, SF6?
A lone pair is attracted by only one nucleus, so it sits closer to the central atom and spreads out more. That extra repulsion squeezes the bonds together.
When explaining a bond angle, always name both the number of bonding pairs and the number of lone pairs, and state that lone pairs repel more than bonding pairs. “Water is bent because of its lone pairs” is not enough for the marks. Also, name the shape from the atoms: NH3 is pyramidal, not tetrahedral, even though its four pairs point towards the corners of a tetrahedron.
Unlocks once the four checks above are done. Worth more XP, written to AS Paper 1 and Paper 2 standard.
What is the shape of the oxonium ion, H3O+?
Which molecule has the smallest bond angle?
Carbon dioxide, O=C=O, is linear because:
Predict the shape of the NH2− ion.
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