Polarisation of ionsEdexcel International A Level Chemistry: Subtopic test

1. 1
   A teacher gives the following ionic radii: Na⁺ 0.102 nm, K⁺ 0.138 nm, Mg²⁺ 0.072 nm and Al³⁺ 0.054 nm. She asks the class to compare how strongly each cation distorts the electron cloud of a neighbouring anion.

   (a)

   Which cation has the greatest polarising power?

   [1 mark]

   • ANa⁺
   • BAl³⁺
   • CMg²⁺
   • DK⁺

   (b)

   What is meant by the polarisation of an anion?

   [1 mark]

   • AThe complete transfer of an electron from the anion to the cation.
   • BThe removal of an outer electron from the anion.
   • CThe attraction between oppositely charged ions in the lattice.
   • DThe distortion of the electron cloud of the anion towards a neighbouring cation.

   (c)

   Explain why Al³⁺ has a greater polarising power than Na⁺.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Lithium forms the halides LiF, LiCl, LiBr and LiI. The ionic radii of the anions are: F⁻ 0.133 nm, Cl⁻ 0.181 nm, Br⁻ 0.196 nm and I⁻ 0.220 nm. The sulfide ion, S²⁻, has a radius of 0.184 nm.

   (a)

   Which anion is the most polarisable?

   [1 mark]

   • AI⁻
   • BBr⁻
   • CCl⁻
   • DF⁻

   (b)

   Which statement best explains why larger anions are more polarisable?

   [1 mark]

   • ATheir nuclei hold the outer electrons more strongly.
   • BThey always carry a greater negative charge.
   • CTheir outer electrons are further from the nucleus and more shielded, so are held less firmly.
   • DThey contain fewer electrons, so the cloud is easier to move.

   (c)

   Explain why the sulfide ion is more polarisable than the chloride ion.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Lithium iodide and sodium iodide are both classed as ionic, but lithium iodide shows more covalent character. Aluminium iodide, AlI₃, has a melting point of only 191 °C, which is low for a compound of a metal with a non-metal. Ionic radii: Li⁺ 0.076 nm, Na⁺ 0.102 nm, Al³⁺ 0.054 nm and I⁻ 0.220 nm.

   (a)

   Explain why the iodide ion is distorted more in lithium iodide than in sodium iodide.

   [3 marks]

   (b)

   Explain, in terms of polarisation, why aluminium iodide has a low melting point for a compound of a metal with a non-metal.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A student is comparing the polarisation in several ionic compounds, using these ionic radii: Li⁺ 0.076 nm, Na⁺ 0.102 nm, K⁺ 0.138 nm, Mg²⁺ 0.072 nm, Al³⁺ 0.054 nm, F⁻ 0.133 nm, Cl⁻ 0.181 nm, S²⁻ 0.184 nm and I⁻ 0.220 nm.

   (a)

   Explain what is meant by polarisation, and how the polarising power of a cation and the polarisability of an anion depend on the radius and charge of the ion, using examples from the data.

   [6 marks]

   (b)

   Place NaF, LiI and MgI₂ in order of increasing polarisation of the anion (increasing covalent character) and justify your order.

   [6 marks]

   Total for question 4: 12 marks

End of questions