Topic 3 · page 6 of 15
Polarisation
A small, highly charged cation can pull on the electron cloud of a large, highly charged anion. The anion is distorted, some electron density lies between the ions, and the bond gains covalent character. Keep polarising power and polarisability apart.
Read first: Ionic radius and bond strength
What polarisation means 3.8
The sodium chloride map on the evidence page shows the ideal ionic model: separate, nearly spherical ions with almost no electron density between them. Real compounds are not always as ideal as that. When a cation sits close to an anion it attracts the outer electrons of the anion.
The steps are:
- The positive cation attracts the outer electrons of the anion towards itself.
- The electron cloud of the anion is no longer spherical. It bulges towards the cation.
- Some electron density now lies between the two ions, so the bond has some covalent character.
Compare this map with the sodium chloride map. There, the density between the ions falls to almost zero. Here, the gap fills in. This is the link to the continuum of bonding in electronegativity and bond polarity.
Polarising power and polarisability 3.9
Two terms are needed, and they belong to different ions.
| Belongs to | What it means | Increases with | |
|---|---|---|---|
| Polarising power | the cation | how strongly it distorts an anion | smaller radius and higher charge (a high charge density) |
| Polarisability | the anion | how easily its cloud is distorted | larger radius and higher charge |
The reasons:
- A cation with a small radius and a high charge has its charge concentrated in a small volume. This charge density is high, so it pulls strongly on the electrons of a nearby anion.
- A large anion has outer electrons that are far from its nucleus and shielded by many inner shells, so they are held loosely and are easy to pull. A higher charge on an anion means more electrons than protons, which also holds the outer electrons less tightly.
Examples:
- Polarising power of the cation: Al³⁺ > Mg²⁺ > Na⁺. The radius falls and the charge rises along the series.
- Li⁺ is more polarising than Na⁺. The charge is the same, so the radius decides.
- Polarisability of the anion: I⁻ > F⁻ (larger radius) and O²⁻ > F⁻ (higher charge).
The most polarisation is found with the smallest, most highly charged cation and the largest, most highly charged anion. A small cation such as Na⁺ with a small anion such as F⁻ gives almost none.
Worked exampleComparing covalent character
Which has more covalent character, NaF or AlI₃?
The cation Al³⁺ (53 pm) is smaller than Na⁺ (102 pm) and has a higher charge (3+ against 1+), so Al³⁺ has the higher charge density and the greater polarising power.
The anion I⁻ (220 pm) is larger than F⁻ (133 pm), so I⁻ is more polarisable.
Both factors point the same way. More distortion of the anion cloud means more electron density between the ions, so AlI₃ has more covalent character than NaF.
Check yourself
- Define polarisation.
- Put Na⁺, Mg²⁺ and Al³⁺ in order of polarising power, largest first, and explain.
- Which is more polarisable, Cl⁻ or Br⁻? Explain.
- State which of the following has more covalent character and explain: LiI or NaI.
- A student writes: "the anion has a large polarising power, so it distorts the cation". Give two corrections.
- Put NaCl, NaI and MgI₂ in order of increasing covalent character, and justify the order.
Answers cover these until you have tried the questions
- Polarisation is the distortion of the electron cloud of an anion by a neighbouring cation.
- Al³⁺ > Mg²⁺ > Na⁺. The three ions have the same 10 electrons. From Na⁺ to Al³⁺ the charge increases (1+, 2+, 3+) and the radius decreases (102, 72 and 53 pm). The charge density rises, so the attraction for the electrons of a nearby anion rises.
- Br⁻. It is larger than Cl⁻ (one more shell), so its outer electrons are further from the nucleus and are held less tightly. They are easier to pull towards a cation.
- LiI. The anion is the same, so the cation decides. Li⁺ and Na⁺ have the same charge, but Li⁺ is smaller, so it has the higher charge density and the greater polarising power. It distorts the I⁻ cloud more, so there is more electron density between the ions.
- (1) It is the cation that polarises, so the anion does not distort anything by polarising power. The correct term for the anion is polarisability. (2) The cloud that is distorted is the one on the anion, not on the cation.
- Order: NaCl < NaI < MgI₂. NaCl to NaI: the cation is the same, but I⁻ is larger than Cl⁻ and so more polarisable, so its cloud is distorted more. NaI to MgI₂: the anion is the same, but Mg²⁺ has a higher charge and a smaller radius than Na⁺, so it has the greater polarising power. The more the anion cloud is distorted, the more electron density lies between the ions and the more covalent character the bond has.