Ammonia (NH₃) is a covalently bonded molecule with a non-linear, trigonal pyramidal shape. Explain why ammonia is described as covalently bonded and explain why it has a trigonal pyramidal shape rather than a flat, triangular shape.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Ammonia has the molecular formula NH₃. Its central nitrogen atom has five electrons in its outer shell.
Model answer (5 marks)
Covalent bonding involves the sharing of a pair of electrons between two atoms. In NH₃ each H atom shares one pair of electrons with the N atom, forming three N–H covalent bonds. The N atom has one lone pair of electrons in addition to the three bonding pairs. The lone pair exerts a greater repulsive force on the bonding pairs than the bonding pairs exert on each other. This increased repulsion pushes the three N–H bonds out of a flat plane, giving the molecule a trigonal pyramidal shape rather than a flat triangular (trigonal planar) shape.
Examiner tips
- Use the term "sharing of a pair of electrons" for covalent bonds
- Mention the lone pair on N
- Explain that lone pair–bonding pair repulsion is stronger than bonding pair–bonding pair repulsion
- Show how this leads to a pyramidal geometry
Common mistakes
- Saying the molecule is linear or that all atoms are in the same plane
- Ignoring the lone pair on nitrogen
- Confusing the shape with tetrahedral rather than trigonal pyramidal
Mark scheme (5 marks)
- Covalent bonding involves the sharing of a pair of electrons between two atoms
- Each hydrogen atom shares one pair of electrons with the nitrogen atom, forming three covalent bonds
- The nitrogen atom has one lone pair of electrons (as well as three bonding pairs)
- The lone pair repels the bonding pairs more strongly than the bonding pairs repel each other
- This greater repulsion pushes the three N–H bonds below a flat plane, giving a trigonal pyramidal shape (rather than a flat triangular/trigonal planar shape)
Key terms in this question
Related
- All Edexcel A-Level Chemistry (9CH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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