# Phosphine (PH₃) and hydrogen sulfide (H₂S) are both covalently bonded molecules. Phosphine has a trigonal pyramidal shape with bond angles of approximately 93°. Hydrogen sulfide has a bent (non-linear) shape with bond angles of approximately 92°. Explain how a covalent bond forms between phosphorus and hydrogen atoms, and explain why phosphine has a trigonal pyramidal shape rather than a flat, triangular (trigonal planar) shape.

> Edexcel A-Level Chemistry (9CH0) — 2.2 Covalent bonding and shapes · Explain · 5 marks

> Phosphorus is in Group 5 of the periodic table and hydrogen is in Group 1. Sulfur is in Group 6 of the periodic table.

## Mark scheme (5 marks)

1. A covalent bond forms by the sharing of a pair of electrons between the phosphorus atom and a hydrogen atom.
2. Both atoms achieve a full outer shell (stable electronic configuration) by sharing electrons.
3. Phosphorus has five electrons in its outer shell, forming three covalent bonds with three hydrogen atoms, leaving one lone pair of electrons on the phosphorus atom.
4. The lone pair of electrons repels the three bonding pairs more strongly than the bonding pairs repel each other.
5. This greater repulsion from the lone pair pushes the three P–H bonding pairs closer together, reducing the bond angle below the tetrahedral angle (below 109.5°), giving a trigonal pyramidal shape.

## Key terms

- [covalent bond](https://www.gradenine.co.uk/glossary/covalent-bond)
- [trigonal pyramidal](https://www.gradenine.co.uk/glossary/trigonal-pyramidal)
- [bond angle](https://www.gradenine.co.uk/glossary/bond-angle)

## Related

- [Revision notes for Edexcel A-Level Chemistry (9CH0)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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Source: [GradeNine](https://www.gradenine.co.uk/q/phosphine-ph-and-hydrogen-sulfide-h-s-d69ddded) · Published by Druglandscape Ltd.