Shapes of molecules and ions

The rule

Electron pairs around the central atom repel each other and get as far apart as possible. Lone pairs repel more than bonding pairs, so each lone pair squeezes the bond angles by about 2.5°. A double bond counts as one region of electron density.

Shapes to know

Predicting a shape

1. Count the outer electrons of the central atom.

2. Add one for each atom bonded by a single bond (each supplies one electron); adjust for charge (add for negative, subtract for positive).

3. Halve to get the pairs; bonds minus pairs gives lone pairs.

4. Name the shape from the positions of the atoms, not the lone pairs.

Worked example

Predict the shape and bond angle of NCl3.

1. N has 5 outer electrons; 3 Cl add 3; total 8 = 4 pairs.
2. 3 bonding pairs and 1 lone pair.

Answer: Pyramidal, about 107°.

Check yourself

1. What is the shape of BF3?
   • Pyramidal
   • Trigonal planar
   • Tetrahedral
   • Bent
   Show answer

   Trigonal planar. B has 3 outer electrons, 3 bonding pairs, no lone pairs.

2. What is the H-O-H bond angle in water, in degrees?
   Show answer

   104.5 °. Two lone pairs squeeze the tetrahedral angle from 109.5° to 104.5°.

3. What is the shape of the ammonium ion, NH4+?
   • Pyramidal
   • Tetrahedral
   • Square planar
   • Trigonal planar
   Show answer

   Tetrahedral. Four bonding pairs and no lone pairs.

4. What is the bond angle in SF6?
   • 109.5°
   • 120°
   • 90°
   • 180°
   Show answer

   90°. Six bonding pairs give an octahedral shape with 90° angles.

5. Why is CO2 linear?
   • There are two regions of electron density (two double bonds) and no lone pairs on carbon
   • Carbon has two lone pairs
   • Oxygen atoms attract each other
   • It has four single bonds
   Show answer

   There are two regions of electron density (two double bonds) and no lone pairs on carbon. Each C=O counts as one region; two regions are furthest apart at 180°.

6. Why is the bond angle in NH3 smaller than in CH4?
   • N is more electronegative than C
   • NH3 has fewer electrons
   • The lone pair on N repels the bonding pairs more strongly than a bonding pair does
   • The N-H bond is longer
   Show answer

   The lone pair on N repels the bonding pairs more strongly than a bonding pair does. Lone pair to bond pair repulsion is greater than bond pair to bond pair.

7. Which bond angles are found in PCl5?
   • 90° and 120°
   • 109.5° only
   • 120° only
   • 90° and 180° only
   Show answer

   90° and 120°. Trigonal bipyramidal: 120° around the middle, 90° to the axial atoms.

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Sources

• Pearson Edexcel IAL Chemistry (2018) scheme of work
• Pearson Edexcel International Advanced Level Chemistry (2018) qualification page

Last updated 7 November 2026. Spotted a mistake? Tell us.