When a force is applied, layers of ions shift by one position.
Like-charged ions end up aligned → strong REPULSION → crystal cracks/shatters.
Cannot bend without breaking — unlike metals.
ELECTRICAL CONDUCTIVITY:
SOLID: Does NOT conduct — ions are in fixed positions in the lattice, cannot move.
MOLTEN (liquid): DOES conduct — ions are free to move and carry charge.
DISSOLVED in water: DOES conduct — ions separate and can move through the solution.
SOLUBILITY:
Many ionic compounds dissolve in WATER — polar water molecules attract and surround the ions, pulling them from the lattice.
Not all are soluble — depends on the strength of ion-ion forces vs. ion-water forces.
Comparing Ionic Compounds — Effect of Charge
The CHARGE on the ions affects the strength of the electrostatic forces:
Higher charge → stronger attraction → higher melting point → higher electrical conductivity when molten.
Comparison:
NaCl (Na⁺ and Cl⁻ — charges of ±1): melting point 801°C.
MgO (Mg²⁺ and O²⁻ — charges of ±2): melting point 2852°C.
MgO has MUCH stronger ionic bonds because both ions have double the charge — the electrostatic force is proportional to the product of the charges.
This also means more energy is released when MgO forms — MgO is a more stable compound.
This concept helps explain why Group 2 compounds often have higher melting points than Group 1 compounds.
⚠️ Common Mistake
Ionic compounds conduct in solution because the IONS move — NOT because electrons move. This is IONIC conduction (different from ELECTRONIC conduction in metals). Also: ionic compounds are brittle, not ductile. They crack when struck because shifting layers brings like charges together → repulsion → shattering.
📌 Key Note
Giant ionic lattice → high MP/BP (strong electrostatic forces). Brittle (layer shift → repulsion → shatters). Solid: no conduction. Molten/dissolved: conducts (mobile ions). Higher ionic charge → stronger forces → higher MP. Many dissolve in water.
🎯 Matching Activity — Ionic vs Metallic Properties
Sort each property into ionic compound or metal. — drag the symbols on the right to match the component names on the left.
Ionic compound
Drop here
Metal
Drop here
Ionic compound
Drop here
Metal
Drop here
Both
Drop here
High melting points — strong electrostatic forces throughout the structure
Malleable — layers of ions can slide without breaking the metallic bond
Brittle — shatters when struck due to repulsion between like-charged ions
Conducts electricity in solid state — delocalised electrons carry charge
Conducts electricity when molten but not when solid
🎯 Test Yourself
Question 1 of 2
1. Magnesium oxide (MgO) has a higher melting point than sodium chloride (NaCl). Why?
2. A student tests whether NaCl conducts electricity in four forms: solid, dissolved in water, molten, and as a gas. In which forms does it conduct?
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