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 · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

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

Model answer (5 marks)

A covalent bond is formed when a pair of electrons is shared between the phosphorus atom and a hydrogen atom, allowing both atoms to attain a full outer shell. Phosphorus has five valence electrons; it forms three single P–H bonds with three hydrogen atoms, using three of its electrons and one electron from each hydrogen. The remaining two electrons on phosphorus form a lone pair.

In a tetrahedral arrangement the four electron pairs (three bonding pairs and one lone pair) would be 109.5° apart. However, a lone pair occupies more space than a bonding pair and exerts greater repulsion. Consequently the three P–H bonding pairs are pushed closer together, reducing the H–P–H bond angle to about 93°. This gives phosphine a trigonal pyramidal shape rather than a flat, triangular (trigonal planar) shape.

Examiner tips

  • Use the term "sharing of a pair of electrons" to describe covalent bonding. Mention the lone pair on phosphorus and its effect on bond angles. Explain the repulsion difference between lone pair–bond pair and bond pair–bond pair. Show the angle reduction from 109.5° to ~93° to justify the pyramidal shape.

Common mistakes

  • Failing to state that the lone pair is on phosphorus. Confusing the bond angle with the tetrahedral angle. Using the wrong term for the shape (e.g., calling it trigonal planar).

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 in this question

covalent bond · trigonal pyramidal · bond angle

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