VSEPR theory and molecular shapes

Key ideas

• Electron group. Any single bond, double bond, triple bond or lone pair on the central atom counts as 1 group.
• Electron geometry is the arrangement of all the groups, lone pairs included.
• Molecular shape describes only where the atoms are. Lone pairs are there, but you do not see them in the shape name.
• Lone pairs squeeze bond angles. They take up more room than bonding pairs, so $\mathrm{NH_3}$ is 107° and $\mathrm{H_2O}$ is 104.5°, not 109.5°.

Use the shorthand $\mathrm{AX_mE_n}$: A is the central atom, X stands for each bonded atom and E for each lone pair on A.

Shape decides polarity

A molecule is polar if its bond dipoles do not cancel. If the central atom has no lone pairs and all the outer atoms are the same, the shape is symmetric and the molecule is nonpolar, even with polar bonds ($\mathrm{CO_2}$, $\mathrm{CH_4}$, $\mathrm{BF_3}$). Lone pairs on the central atom ($\mathrm{H_2O}$, $\mathrm{NH_3}$) or different outer atoms ($\mathrm{CH_3Cl}$) usually make it polar. Two exceptions in the chart: $\mathrm{XeF_2}$ and $\mathrm{XeF_4}$ have lone pairs but are symmetric, so they are nonpolar.

Worked examples

Common mistakes and how to fix them

• Skipping the Lewis structure. You cannot count lone pairs without it. Fix: draw it first, see how to draw Lewis structures.
• Counting a double bond as 2 groups. Fix: any bond between two atoms is 1 group, single, double or triple.
• Naming the electron geometry instead of the shape. Water's electron geometry is tetrahedral, but its shape is bent. Fix: name the shape from the atoms only.
• Counting lone pairs on outer atoms. Fix: only lone pairs on the central atom change the shape.
• Calling every molecule with polar bonds polar. Fix: check whether the shape is symmetric.