What is polarising power?

The ability of a cation to distort the electron cloud of a neighbouring anion.

What two factors determine the polarising power of a cation?

Its charge (higher is greater) and its radius (smaller is greater).

What is polarisability?

How easily the electron cloud of an anion is distorted.

What two factors determine the polarisability of an anion?

Its radius (larger is more polarisable) and its charge (more negative is more polarisable).

Which has the greater polarising power, Li⁺ or K⁺?

Li⁺, because it is smaller and has the same charge, so a higher charge density.

Which is more polarisable, I⁻ or F⁻?

I⁻, because the outer electrons are further from the nucleus and more shielded.

Why is Al³⁺ strongly polarising?

It has a high charge (3+) and a very small radius, so a very high charge density.

What does a high degree of polarisation produce?

Electron density between the nuclei, so the bonding has some covalent character.

Why is S²⁻ more polarisable than Cl⁻?

It has a higher charge and fewer protons holding the same 18 electrons, so its electrons are held less strongly.

What combination of ions gives most covalent character?

A small, highly charged cation with a large, highly charged anion.

What is charge density?

The charge on an ion relative to its size.

Polarisation of ionsEdexcel International A Level Chemistry: Flashcards

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What is polarisation of an ion?

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What is polarisation of an ion?: The distortion of the electron cloud of an anion by a neighbouring cation.
What is polarising power?: The ability of a cation to distort the electron cloud of a neighbouring anion.
What two factors determine the polarising power of a cation?: Its charge (higher is greater) and its radius (smaller is greater).
What is polarisability?: How easily the electron cloud of an anion is distorted.
What two factors determine the polarisability of an anion?: Its radius (larger is more polarisable) and its charge (more negative is more polarisable).
Which has the greater polarising power, Li⁺ or K⁺?: Li⁺, because it is smaller and has the same charge, so a higher charge density.
Which is more polarisable, I⁻ or F⁻?: I⁻, because the outer electrons are further from the nucleus and more shielded.
Why is Al³⁺ strongly polarising?: It has a high charge (3+) and a very small radius, so a very high charge density.
What does a high degree of polarisation produce?: Electron density between the nuclei, so the bonding has some covalent character.
Why is S²⁻ more polarisable than Cl⁻?: It has a higher charge and fewer protons holding the same 18 electrons, so its electrons are held less strongly.
What combination of ions gives most covalent character?: A small, highly charged cation with a large, highly charged anion.
What is charge density?: The charge on an ion relative to its size.

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

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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).