Shapes of simple molecules and ionsAQA A-Level Chemistry: Subtopic test

1. 1
   Boron trifluoride, BF₃, is a gas used as a catalyst in organic synthesis. It reacts with fluoride ions to form the tetrafluoroborate ion, BF₄⁻, in which boron is bonded to four fluorine atoms.

   (a)

   What is the F–B–F bond angle in a molecule of BF₃?

   [1 mark]

   • A107°
   • B109.5°
   • C120°
   • D180°

   (b)

   What is the shape of the BF₄⁻ ion?

   [1 mark]

   • ATetrahedral
   • BSquare planar
   • CPyramidal
   • DTrigonal planar

   (c)

   Explain why BF₃ and BF₄⁻ have different shapes.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Sulfur forms several fluorides. Sulfur hexafluoride, SF₆, is an unreactive gas used as an insulator in electrical switchgear. Sulfur tetrafluoride, SF₄, is a reactive gas used to make fluorinated organic compounds. Sulfur has six electrons in its outer shell.

   (a)

   What is the shape of an SF₆ molecule?

   [1 mark]

   • ASquare planar
   • BTrigonal bipyramidal
   • CTetrahedral
   • DOctahedral

   (b)

   What is the F–S–F bond angle between adjacent fluorine atoms in SF₆?

   [1 mark]

   • A120°
   • B90°
   • C109.5°
   • D180°

   (c)

   Deduce the number of bonding pairs and lone pairs of electrons around sulfur in SF₄ and name the shape of the molecule.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Phosphorus forms two chlorides, PCl₃ and PCl₅. Phosphorus has five electrons in its outer shell. In the vapour phase both compounds exist as separate molecules.

   (a)

   Deduce the shape of a PCl₃ molecule and explain why the Cl–P–Cl bond angle is about 107° rather than 109.5°.

   [3 marks]

   (b)

   Explain why a PCl₅ molecule has two different bond angles and state their values.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A teacher asks students to compare the shapes of methane, CH₄, ammonia, NH₃, and water, H₂O, and of two unfamiliar molecules, xenon difluoride, XeF₂, and xenon tetrafluoride, XeF₄. Xenon has eight electrons in its outer shell.

   (a)

   Deduce the shapes of XeF₂ and XeF₄, explaining how electron pair repulsion determines each shape and the bond angles.

   [6 marks]

   (b)

   Compare the shapes and bond angles of CH₄, NH₃ and H₂O and explain the trend in their bond angles.

   [6 marks]

   Total for question 4: 12 marks

End of questions