Topic 3 · page 12 of 15
Shapes with no lone pairs
When every pair on the central atom is a bonding pair, the shape depends only on the number of regions, from two (linear) to six (octahedral). Learn the five shapes in this group, then use the counting method to predict new ones.
Read first: Electron-pair repulsion theory
Two and three regions 3.18
With no lone pairs, the pairs spread out as far as possible, and the shape follows the number of regions.
| Regions | Shape | Bond angle |
|---|---|---|
| 2 | Linear | 180° |
| 3 | Trigonal planar | 120° |
| 4 | Tetrahedral | 109.5° |
| 5 | Trigonal bipyramidal | 120° and 90° |
| 6 | Octahedral | 90° |
- BeCl₂: Be has 2 outer electrons and each Cl adds 1, so 2 + 2 = 4, which is 2 pairs, both bonding. The pairs are as far apart as possible: linear, 180°.
- CO₂: carbon has two bonding regions, the two C=O double bonds. The two regions are as far apart as possible: linear, 180°.
- BCl₃: B has 3 and each Cl adds 1, so 3 + 3 = 6, which is 3 pairs, all bonding. Trigonal planar, 120°.
- C₂H₄: each carbon has three regions (two C–H bonds and one C=C). The shape at each carbon is trigonal planar, with bond angles of about 120°, and the whole molecule is flat.
Four regions: tetrahedral 3.18
- CH₄: 4 + 4 = 8, which is 4 pairs, all bonding. Tetrahedral, 109.5°.
- NH₄⁺: 5 + 4 − 1 = 8, which is 4 pairs, all bonding. Tetrahedral, 109.5°.
CH₄ and NH₄⁺ have different formulas but the same shape and angle. Both have four bonding pairs and no lone pairs, so the repulsion between the pairs is equal.
Five and six regions 3.18
- PCl₅ (gas): P has 5 outer electrons and each Cl adds 1, so 5 + 5 = 10, which is 5 pairs, all bonding. The shape is trigonal bipyramidal. Three chlorine atoms lie in a plane at 120° to each other. The other two lie above and below that plane, at 90° to the plane. Give both angles.
- SF₆: 6 + 6 = 12, which is 6 pairs, all bonding. The shape is octahedral, and the bond angle between neighbouring bonds is 90°.
The specification asks for PCl₅ as a gas. This is the form with the simple molecules. Quote the shape for the gas.
Predicting by analogy 3.19
Specification point 3.19 asks you to use the theory on species you have not seen. Each one below is analogous to a species above. Use the counting method, then name the shape and angle.
| Species | Counting | Pairs | Shape | Angle | Like |
|---|---|---|---|---|---|
| SiCl₄ | 4 + 4 = 8 | 4 bonding | Tetrahedral | 109.5° | CH₄ |
| BF₃ | 3 + 3 = 6 | 3 bonding | Trigonal planar | 120° | BCl₃ |
| CS₂ | two C=S regions | 2 regions | Linear | 180° | CO₂ |
| PF₅ | 5 + 5 = 10 | 5 bonding | Trigonal bipyramidal | 120° and 90° | PCl₅ |
| SiF₆²⁻ | 4 + 6 + 2 = 12 | 6 bonding | Octahedral | 90° | SF₆ |
| PCl₄⁺ | 5 + 4 − 1 = 8 | 4 bonding | Tetrahedral | 109.5° | CH₄ |
The ions test the charge step: add 2 for the 2− ion SiF₆²⁻ and subtract 1 for the 1+ ion PCl₄⁺.
Check yourself
- Explain the shape of BCl₃ and state its bond angle.
- Explain why CO₂ is linear.
- State the shape of gaseous PCl₅, give both bond angles, and say which atoms they are between.
- Predict the shapes and bond angles of SiF₆²⁻ and PCl₄⁺, showing the electron count.
- Predict the shape and bond angle of BeF₂ and of SiH₄, giving the species each resembles.
- CH₄ and NH₄⁺ have the same bond angle. Explain why.
Answers cover these until you have tried the questions
- B has 3 outer electrons and each of 3 Cl adds 1, which gives 6 electrons, or 3 pairs. All three are bonding pairs and there are no lone pairs. The pairs repel and get as far apart as possible, so the shape is trigonal planar with a bond angle of 120°.
- Carbon has two C=O double bonds. Each double bond counts as one bonding region, so there are two regions and no lone pairs. The regions repel and get as far apart as possible, giving a linear molecule with a bond angle of 180°.
- Trigonal bipyramidal. There are 120° angles between the three chlorine atoms in the plane, and 90° angles between each of those three and the two chlorine atoms above and below the plane.
- SiF₆²⁻: 4 + 6 + 2 = 12, which is 6 bonding pairs, so octahedral, 90°. PCl₄⁺: 5 + 4 − 1 = 8, which is 4 bonding pairs, so tetrahedral, 109.5°.
- BeF₂ is like BeCl₂: Be has 2 + 2 = 4 electrons, which is 2 bonding pairs, so linear, 180°. SiH₄ is like CH₄: 4 + 4 = 8, which is 4 bonding pairs, so tetrahedral, 109.5°.
- Both have four bonding pairs and no lone pairs round the central atom. The pairs repel equally and get as far apart as possible, so both are tetrahedral with bond angles of 109.5°.