Polarisation of ionsEdexcel International A Level Chemistry: Revision notes
Section 1
What polarisation means
In a perfectly ionic model, ions are spheres of charge. In real compounds the positive charge of a cation attracts the electron cloud of a neighbouring anion and pulls it out of shape. This distortion is called polarisation.
The more the anion's electron cloud is drawn towards the cation, the more electron density lies between the two nuclei, and the more the bonding takes on some covalent character.
Polarisation is a distortion of the anion's electron cloud, not a transfer of electrons or the removal of an electron.
Section 2
Polarising power of a cation
The polarising power of a cation is its ability to distort the electron cloud of an anion. It depends on two things.
- Charge: a higher charge attracts the electron cloud more strongly.
- Radius: a smaller radius concentrates the charge, giving a higher charge density.
So small, highly charged cations such as Al³⁺ (0.054 nm, 3+) have the greatest polarising power, while large, singly charged cations such as K⁺ (0.138 nm, 1+) have the least. Mg²⁺ (0.072 nm, 2+) is greater than Li⁺ (0.076 nm, 1+) because of its charge.
Always give both factors, charge and radius, when you compare cations, and use the data to show which is larger or smaller.
Section 3
Polarisability of an anion
The polarisability of an anion is how easily its electron cloud is distorted. It also depends on two things.
- Radius: in a large anion the outer electrons are far from the nucleus and more shielded, so they are held less firmly. I⁻ (0.220 nm) is more polarisable than F⁻ (0.133 nm).
- Charge: a higher negative charge means the same number of electrons is held by fewer protons, so the cloud is easier to distort. S²⁻ is more polarisable than Cl⁻ although the radii are similar.
Section 4
Predicting and comparing
To compare the polarisation in two compounds, work through three steps.
- Rank the cations by polarising power: high charge and small radius is greatest.
- Rank the anions by polarisability: large radius and high charge is greatest.
- The most polarisation, and so the most covalent character, occurs with a highly polarising cation and a highly polarisable anion.
Example: MgI₂ shows more polarisation than LiI, which shows more than NaF. Mg²⁺ has twice the charge of Li⁺ and a similar radius; I⁻ is far more polarisable than F⁻.
Do not say a large cation polarises more. A large cation has lower charge density, so a smaller polarising power.
Must know
- Polarisation is the distortion of an anion's electron cloud by a cation
- Polarising power increases with cation charge and decreases with radius
- Polarisability increases with anion radius and anion charge
- A highly polarising cation with a highly polarisable anion gives the most covalent character
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Polarisation of ions
- 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.Explain why Al³⁺ has a greater polarising power than Na⁺.2 marks
- 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.Explain why the sulfide ion is more polarisable than the chloride ion.2 marks
- 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.Explain why the iodide ion is distorted more in lithium iodide than in sodium iodide.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).