Explain ionic polarisation in Group 2 compounds
Connect cation size to polarising power and to the distortion of a large anion.
A Level; Cambridge International Chemistry 9701, 2028–2030 syllabus, Topic 27. Compare Group 2 ions at the same charge and distinguish lattice formation, lattice dissociation, hydration and solution enthalpy. Trend explanations are qualitative unless data are supplied. Written practice is self-checked
Before you begin
- Use AS Group 2 reactivity trends, ionic charge and radius, Hess cycles, and balanced equations.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which factor increases the polarising power of a 2+ ion?
What you will learn
- Relate cation radius to charge density and polarising power
- Explain how a cation distorts a large anion
- Connect anion polarisation to thermal decomposition
A smaller 2+ ion has greater polarising power
Across Group 2, each cation has charge 2+, but the ionic radius increases from Mg²⁺ to Ba²⁺. The smaller Mg²⁺ ion has the greater charge density and attracts the electron cloud of a neighbouring anion more strongly.
Polarising power describes the cation's ability to distort an anion's electron cloud. Keep the comparison controlled: the ions have the same charge, so the changing radius explains the difference.
Distortion weakens the large anion
A large anion such as CO₃²⁻ or NO₃⁻ is more readily polarised than a small anion. A small cation distorts its electron cloud, making the anion less stable and helping it break down when heated.
Down Group 2 the cation becomes larger and its polarising power decreases. The carbonate or nitrate anion is less distorted, so a higher temperature is needed to decompose the compound. This is a qualitative explanation, not a rule that every salt has a single universal decomposition temperature.
Pause and explain
What does a small Group 2 cation do to a large carbonate ion?
Worked example
Explain why Mg²⁺ polarises CO₃²⁻ more strongly than Ba²⁺ does.
Show the worked solution
- Both cations have charge 2+.
- Mg²⁺ has the smaller radius and higher charge density.
- Its stronger attraction distorts the carbonate electron cloud more.
Answer Mg²⁺ is smaller than Ba²⁺ but has the same charge, so it has greater charge density and stronger polarising power. It distorts the large CO₃²⁻ ion more strongly.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Rank Mg²⁺, Ca²⁺ and Ba²⁺ from greatest to least polarising power.
Give me a hint
All have the same charge; compare their ionic radii down the group.
Compare my reasoning
- The cation charge remains 2+.
- Ionic radius increases from Mg²⁺ to Ba²⁺.
- Smaller radius at equal charge means greater charge density.
Mg²⁺ > Ca²⁺ > Ba²⁺ in polarising power.
Look for these in your work
- Charge held constant
- Radius trend correct
- Order correctly inferred
A learner says Sr²⁺ polarises NO₃⁻ more than Mg²⁺ because strontium has more protons. Correct the reasoning.
Give me a hint
Compare the charge and ionic radius of the cations, not proton number alone.
Compare my reasoning
- Both ions carry a 2+ charge.
- Sr²⁺ is larger than Mg²⁺.
- At the same charge, the smaller Mg²⁺ has greater charge density.
Mg²⁺ has greater polarising power: both ions have charge 2+, but Mg²⁺ is smaller. Proton number alone does not determine the polarising power in this comparison.
Look for these in your work
- Equal ionic charge stated
- Ionic size compared
- Conclusion linked to charge density
Two hypothetical 2+ cations have radii 70 pm and 135 pm. Which should distort a large 2− anion more, and what is the limitation of this prediction?
Give me a hint
Use radius to compare charge density, then state that this is a qualitative model.
Compare my reasoning
- The smaller ion concentrates the same charge into less space.
- Greater charge density gives stronger polarising power.
- Do not infer an exact bond type or decomposition temperature from radius alone.
The 70 pm ion should polarise the large anion more strongly because it has the same 2+ charge at smaller radius. This qualitative comparison does not by itself give a numerical decomposition temperature or complete bonding description.
Look for these in your work
- Same charge recognised
- Smaller-ion effect explained
- Prediction limitation stated
Common mistakes
- Saying the cation charge increases down Group 2
- Explaining the trend using atomic radius rather than ionic radius
- Claiming the anion polarises the cation in this comparison
- Treating polarisation as a change in the formal ionic charge
What can you explain now?
Which factor increases the polarising power of a 2+ ion?
Compare with the explanation
A smaller ionic radius
The same charge concentrated into a smaller ion gives greater charge density.
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Make it stick
At equal charge, how does ionic radius affect charge density and polarising power?
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