The questions are on the VSEPR theory test. Take it under real conditions before reading the mark scheme.
Quick answer: Molecular shape follows VSEPR theory: electron domains around the central atom repel and spread as far apart as possible, with lone pairs repelling more than bonding pairs. This mark scheme gives the model answers and the marks for the 8 questions of the Chemistry 11-12 test on VSEPR theory, from shape of beryllium chloride to boron trifluoride and ammonia, with the equivalent answers to accept.
Marks are given for the correct shape or value and for the reason. Explanations must mention lone pairs, symmetry or electronegativity as appropriate. Accept equivalent wording and bond angles within about two degrees.
Level: Chemistry grades 11-12 (A Level · IB · Class 11-12) · Curriculum: IB S2.2
Question 1 Shape of beryllium chloride (2 marks)
The molecule is linear with an angle of 180°.Marking: 1 for the shape; 1 for the angle.
Question 2 Water compared with methane (3 marks)
Both have four electron domains in a tetrahedral arrangement. Water has two lone pairs and methane has none. Lone pairs repel more than bonding pairs, so the bond angle in water is smaller.Marking: 1 for four domains; 1 for lone pairs in water; 1 for greater repulsion.
Question 3 A non-polar molecule with polar bonds (3 marks)
Each C–X bond is polar because X is more electronegative than C, so the dipole points toward X. The molecule is a symmetrical tetrahedron, so the four equal dipoles cancel and the net dipole is zero.Marking: 1 for polar bonds (difference in electronegativity); 1 for symmetry; 1 for cancellation.
Question 4 Bond polarity (2 marks)
C–H = 0.4; C–O = 3.4 − 2.6 = 0.8; C–F = 4.0 − 2.6 = 1.4. Order of increasing polarity: C–H < C–O < C–F.Marking: 1 for three correct differences; 1 for the order.
Question 5 Ammonium and ammonia (3 marks)
NH₄⁺ has four bonding pairs and is tetrahedral, 109.5°. NH₃ has three bonding pairs and one lone pair: trigonal pyramidal, about 107°. The lone pair repels more strongly.Marking: 1 for each shape with angle; 1 for the lone pair explanation.
Question 6 Sulfur dioxide polarity (2 marks)
It is polar. The S=O bonds are polar and, because the molecule is bent, the dipoles do not cancel.Marking: 1 for polar; 1 for the shape reason.
Question 7 Hydrogen iodide (2 marks)
Difference = 2.7 − 2.2 = 0.5. Iodine is more electronegative, so hydrogen carries δ+.Marking: 1 for the difference; 1 for δ+ on H.
Question 8 Boron trifluoride and ammonia (3 marks)
BF₃ has three bonding pairs and no lone pairs, so it is trigonal planar and symmetrical, and the B–F dipoles cancel. NH₃ has a lone pair, so it is pyramidal and the N–H dipoles do not cancel.Marking: 1 for the BF₃ shape; 1 for NH₃ shape or lone pair; 1 for cancellation contrast.
