Shapes of molecules

AS Level Chemistry · Chemical bonding. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.

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The idea

The shape of a molecule is decided by the pairs of electrons around its central atom. The pairs repel each other and move as far apart as they can. This is VSEPR theory (valence shell electron pair repulsion).

First count the electron pairs around the central atom, then sort them into bonding pairs and lone pairs. A double or triple bond counts as one region.

Lone pairs are held closer to the central atom, so they repel more strongly than bonding pairs. Each lone pair squeezes the bond angle by about 2.5°. The shape is named from the positions of the atoms only, not the lone pairs.

Rules to remember

Common mistake

Students give the shape from the formula alone, so they call NH3 trigonal planar. Always count the lone pairs on the central atom first.

Worked example

What are the shape and bond angle of the AlCl4− ion?

  1. Aluminium has 3 outer electrons. Add 4 from the four chlorine atoms and 1 for the negative charge: 3 + 4 + 1 = 8 electrons.
  2. 8 electrons = 4 pairs, and all four are bonding pairs (no lone pairs).
  3. Four bonding pairs repel equally, as in methane.

Answer: Tetrahedral, with bond angles of 109.5°.

Practice questions

Try each one, then open the answer.

1. Why is the bond angle in water (104.5°) smaller than the bond angle in methane (109.5°)?

  1. A
    oxygen is more electronegative than carbon
  2. B
    the O–H bond is shorter than the C–H bond
  3. C
    water has fewer electron pairs around its central atom
  4. D
    water has two lone pairs, which repel more strongly than bonding pairs and squeeze the O–H bonds closer together
Show answer

Answer: D. Both molecules have four electron pairs. In water two of them are lone pairs; lone pair–lone pair repulsion is greater than bonding pair repulsion, so the H–O–H angle is compressed.

2. What is the shape of the amide ion, NH2−?

  1. A
    non-linear (bent), with a bond angle of about 104.5°
  2. B
    linear, 180°
  3. C
    pyramidal, about 107°
  4. D
    trigonal planar, 120°
Show answer

Answer: A. Nitrogen in NH2− has 5 + 2 + 1 = 8 outer electrons: two bonding pairs and two lone pairs, exactly like water. The shape is therefore bent, with an angle close to 104.5°.

3. What is the H–N–H bond angle in ammonia?

  1. A
    104.5°
  2. B
    107°
  3. C
    109.5°
  4. D
    120°
Show answer

Answer: B. Ammonia has three bonding pairs and one lone pair. The lone pair repels more strongly than bonding pairs, pushing the bonds from the tetrahedral 109.5° down to about 107°.

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