The questions are on the vsepr theory test. Take it under real conditions before reading the mark scheme.
Quick answer: A Lewis structure shows how valence electrons are shared as bonds and lone pairs, and formal charges help choose the best one. This mark scheme gives the model answers and the marks for the 8 questions of the AP Chemistry test on vsepr theory, from Lewis structure of PCl₃ to spot the error, with the equivalent answers to accept.
Marks reward correct electron counting, the right domain count and clear reasoning about polarity. Accept equivalent wording or a correctly drawn structure instead of a description.
Level: AP Chemistry · Curriculum: AP Chem Unit 2
Question 1 Lewis structure of PCl₃ (3 marks)
- P in the centre with three single bonds to Cl, each Cl with three lone pairs, and one lone pair on P.
- Trigonal pyramidal, about 100° to 107°.
- Four domains: sp³.
Marking: 1 mark each part.
Question 2 Formal charges in carbon monoxide (3 marks)
- C: 4 − 2 − 3 = −1.
- O: 6 − 2 − 3 = +1.
- Both atoms have an octet, which is more important, and the triple bond gives a very strong bond. The formal charges are small.
Marking: 1 mark each part.
Question 3 Resonance in carbonate (2 marks)
- Three.
- (2 + 1 + 1)/3 = 1.33.
Marking: 1 mark each part.
Question 4 Sulfur tetrafluoride (3 marks)
- Four bonds use 8 electrons, the F atoms have 24, so 2 remain: one lone pair.
- Five domains: electron geometry trigonal bipyramidal, molecular shape seesaw.
- Polar: the lone pair makes the shape asymmetric, so the dipoles do not cancel.
Marking: 1 mark each part.
Question 5 Methanal (3 marks)
- 2 C–H σ, 1 C=O σ and 1 π: 3 σ and 1 π.
- Three domains: sp².
- Slightly less than 120°, because the double bond region has more electron density and repels the C–H bonds more.
Marking: 1 mark each part.
Question 6 Polarity of CF₄ and CHF₃ (2 marks)
CF₄ is tetrahedral and the four identical C–F dipoles cancel, so it is nonpolar. In CHF₃ one F is replaced by H, so the dipoles are unequal and do not cancel; the molecule is polar.Marking: 1 mark for symmetry cancellation; 1 mark for the asymmetry of CHF₃.
Question 7 Bond strength (2 marks)
- The C=O bond in methanal.
- The double bond shares more electron pairs, so the nuclei are pulled closer together.
Marking: 1 mark each part.
Question 8 Spot the error (2 marks)
Nitrogen has a lone pair as well as three bonds, so it has four domains and tetrahedral electron geometry. The molecule is trigonal pyramidal with an angle of about 107°.Marking: 1 mark for noting the lone pair; 1 mark for the shape and angle.
