1.6.4Unit 1. Structure, Bonding and Introduction to Organic Chemistry
Polarisation — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions
5 min read 5 parts Lesson 20 of 36 Unit quiz
Part 1 of 5The Idea of Polarisation
Many ionic compounds are not purely ionic because the ions are not perfect spheres.
Perfect ionic model
- This assumes spherical ions whose charge is centred on the nucleus and whose electron clouds do not overlap.
Cation effect
- A small cation with high charge attracts the outer electrons of a neighbouring anion towards itself.
Distortion
- The anion electron cloud is pulled out of shape, so some electron density lies between the two nuclei like a partial covalent bond.
Polarising power
- The ability of a cation to distort an anion, which increases with the charge density of the cation.
Part 2 of 5Factors That Increase Polarisation
Factors That Increase Polarisation
High cation charge
- Al³⁺ distorts anions much more than Na⁺ because it has three positive charges in a small volume.
Small cation radius
- Li⁺ is smaller than Na⁺ so it polarises anions more, giving lithium compounds more covalent character.
Large anion
- Big anions such as I⁻ have outer electrons far from the nucleus and are easily distorted, so they are highly polarisable.
High anion charge
- An anion with a greater negative charge, such as S²⁻, is held less tightly and also polarises more easily than a halide.
Part 3 of 5Evidence and Consequences
1
Aluminium chloride behaves like a covalent compound, forming molecules that sublime at low temperature, because Al³⁺ polarises Cl⁻ strongly.
2
Experimental lattice enthalpies are compared with those calculated for a perfect ionic model.
3
When the experimental value is more exothermic than the calculated one, the extra stabilisation indicates covalent character.
4
The difference gets bigger as the cation becomes smaller and more highly charged and as the anion becomes larger.
5
Sodium chloride agrees closely with the model, while silver iodide differs noticeably.
Part 4 of 5Using the Idea in Answers
Using the Idea in Answers
Comparing two salts
- Say which cation has the higher charge density and which anion is larger, then link both to the extent of distortion.
Covalent character
- Describe it as electron density shared between the nuclei, rather than saying the bond is covalent.
Trend
- Covalent character increases from LiF to LiI because the larger iodide ion is more easily polarised.
Not a separate bond
- Polarisation is a modification of ionic bonding, so the compound is still a lattice of ions.
| Cation | Charge | Polarising power |
|---|---|---|
| Na⁺ | 1+ | Low |
| Mg²⁺ | 2+ | Medium |
| Al³⁺ | 3+ | High |
Part 5 of 5Exam Checklist & Tips
1.6.4 – Key Points for the Exam
- Polarisation Distortion of the electron cloud of an anion by a nearby cation.
- Polarising power The ability of a cation to distort an anion, greater for small highly charged cations.
- Polarisability How easily an anion electron cloud is distorted, greater for large anions.
- Covalent character Partial sharing of electron density between ions in a nominally ionic compound.
- Charge density The charge of an ion divided by its size.
Common Mistakes
- Stating that a large cation has high polarising power; it is small cations that distort most.
- Saying the anion polarises the cation instead of the reverse.
- Describing the compound as covalent rather than as having partial covalent character.
- Forgetting that polarisability increases with anion size.
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