Read Fajan's Rule Before Ranking Ionic vs Covalent Character
Most 'which order is correct' questions about melting point, solubility, thermal stability or conductivity trace back to one idea — Fajan's rule: small, highly-charged cations polarise large, easily-deformed anions, pulling a bond away from purely ionic toward covalent character.
Chemistry questions first require the material, particles or reaction condition to be identified before a formula or rule is selected.
Fajan's rule in one sentence
Ionic character increases with a LARGE cation, SMALL charge on the cation, and a SMALL anion (the opposite combination — small highly-charged cation with a large anion — maximises covalent character instead, since the small dense cation polarises the large anion's electron cloud). Apply this before ranking lattice energy, melting point, or solubility for any ionic-vs-covalent comparison.
Lattice energy and hydration energy both scale with charge density
Higher charge density (smaller ionic radius, higher charge) means both higher lattice energy (stronger ionic attraction in the solid) AND higher hydration energy (stronger ion-dipole attraction with water) — Be²⁺ has both the highest lattice energy and highest hydration energy among group 2 ions for exactly this reason, and comparing Be²⁺ to Li⁺ needs both effects tracked together, not just one.
Thermal stability of carbonates, sulfates and nitrates tracks cation polarising power
A smaller, more polarising cation destabilises the large carbonate/sulfate/nitrate anion toward decomposition — this is why thermal stability of carbonates INCREASES down group 2 (Be < Mg < Ca < Sr < Ba) as the cation grows larger and less polarising, and why lithium carbonate (small Li⁺) is markedly less thermally stable than the other alkali carbonates.
Solubility trends split by which lattice-energy vs hydration-energy effect dominates
Group 2 hydroxide solubility INCREASES down the group (lattice energy falls off faster than hydration energy as the cation grows) while group 2 sulfate solubility DECREASES down the group (here hydration energy falls off faster) — the same 'bigger cation' trend can point in opposite solubility directions depending on which anion is involved, so don't apply one solubility rule universally across every anion.
Electrical conductivity and ionic mobility need actual ion mobility, not just ionic character
A compound can be ionic in bonding character yet non-conducting as a solid because ions can't physically move through a rigid lattice — conductivity in the solid state requires melting or dissolving to free the ions, which is why fused (molten) salts conduct while the same solid salt doesn't.
OlympIQ exam lens
- ✓Fajan's rule: small cation + high cation charge + large anion → more covalent character; large cation + low charge + small anion → more ionic character.
- ✓Higher charge density raises both lattice energy and hydration energy together — track both effects for the same ion.
- ✓Thermal stability of carbonates/sulfates/nitrates rises as the cation grows larger (less polarising) down a group.
- ✓The SAME 'bigger cation down the group' trend can raise solubility for one anion (hydroxides) while lowering it for another (sulfates) — check which energy term dominates for that specific anion.
- ✓Ionic solids don't conduct electricity until molten or dissolved — ionic bonding alone isn't enough, ion mobility is required.
Don't apply a single 'bigger cation = more soluble' or 'bigger cation = more stable' rule across every anion type — hydroxide solubility and sulfate solubility move in OPPOSITE directions down group 2, because a different one of lattice energy vs hydration energy dominates for each anion.
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