Shapes of molecules and ionsEdexcel A-Level Chemistry: Subtopic test

1. 1
   Boron trichloride, BCl₃, is a gas used in the manufacture of semiconductors. It reacts with ammonia, NH₃, to form a solid adduct, H₃N→BCl₃, in which the boron atom is bonded to four atoms.

   (a)

   What is the shape of a BCl₃ molecule?

   [1 mark]

   • APyramidal
   • BTetrahedral
   • CBent
   • DTrigonal planar

   (b)

   What is the approximate bond angle around the boron atom in H₃N→BCl₃?

   [1 mark]

   • A109.5°
   • B120°
   • C107°
   • D90°

   (c)

   Explain why the Cl–B–Cl bond angle changes from 120° in BCl₃ to about 109.5° in the adduct.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   The H–X–H bond angles in three hydrides of second-row elements are measured as 109.5° in methane, CH₄, 107° in ammonia, NH₃, and 104.5° in water, H₂O.

   (a)

   What is the shape of a water molecule?

   [1 mark]

   • ALinear
   • BTrigonal planar
   • CBent
   • DTetrahedral

   (b)

   Which row correctly shows the repulsion between pairs of electrons, strongest first?

   [1 mark]

   • Abonding pair/bonding pair > lone pair/bonding pair > lone pair/lone pair
   • Blone pair/lone pair > lone pair/bonding pair > bonding pair/bonding pair
   • Clone pair/bonding pair > lone pair/lone pair > bonding pair/bonding pair
   • Dlone pair/lone pair > bonding pair/bonding pair > lone pair/bonding pair

   (c)

   Explain why the bond angle in water is smaller than the bond angle in ammonia.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Sulfur hexafluoride, SF₆, is an unreactive gas used to insulate electrical switchgear. Phosphorus pentachloride, PCl₅, is a reagent used in organic chemistry. Both are simple molecules in the gas phase. Phosphorus has five outer electrons, sulfur has six and bromine has seven.

   (a)

   Deduce the shape of a gaseous PCl₅ molecule and the bond angles in it, explaining your answer.

   [3 marks]

   (b)

   Deduce the shape of SF₆ and the shape of BrF₅, which has five fluorine atoms bonded to one bromine atom. Explain each shape and give the approximate bond angle.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Xenon, a noble gas, forms fluorides. Xenon difluoride, XeF₂, is a linear molecule and xenon tetrafluoride, XeF₄, is square planar. Sulfur tetrafluoride, SF₄, is also known. A xenon atom has eight outer electrons and a sulfur atom has six, and in each molecule the fluorine atoms are joined to the central atom by single bonds.

   (a)

   Use electron-pair repulsion theory to explain the shapes of XeF₂ and XeF₄.

   [6 marks]

   (b)

   Predict the shape of SF₄ and explain it, including how the bond angles compare with those in a regular shape.

   [6 marks]

   Total for question 4: 12 marks

End of questions