IB Chemistry SL S2.2 The covalent model questions

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Question 1

Multiple choiceMedium1 mark

For molecules whose central atoms have four electron domains, the graph shows mean bond angle against the number of lone pairs on the central atom.

What is the average decrease in mean bond angle per added lone pair from 0 to 2 lone pairs, and the molecular geometry at 2 lone pairs?

Show answer and explanation

Answer: A. The average decrease is (109.5° − 104.5°) / (2 − 0) = 2.5° per added lone pair. With four electron domains and two lone pairs, the molecular geometry is bent.

B: subtracts the endpoint angles but does not divide by the two added lone pairs, giving 5.0° per lone pair.
C: assigns the trigonal-pyramidal geometry associated with one lone pair rather than the bent geometry for two.
D: combines the undivided angle change with the one-lone-pair geometry.

Remember: molecular geometry describes the arrangement of atoms, not lone pairs.

Question 2

Paper 2Medium4 marks

Figure 1 shows Lewis representations of ammonia, $\ce{NH3}$ and methane, $\ce{CH4}$ Each line represents a single covalent bond. The arrangements shown do not represent the three-dimensional molecular shapes.

(a)

State the electron-domain geometry around the central atom in each molecule.

[1]

(b)

Compare the molecular geometries of $\ce{NH3}$ and $\ce{CH4}$

[1]

(c)

The H–N–H bond angle in $\ce{NH3}$ is smaller than the H–C–H bond angle in $\ce{CH4}$ Explain why.

[2]

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