When ionic compounds form, the positive and negative ions don't just stay in pairs — they arrange themselves into a GIANT IONIC LATTICE.
A giant ionic lattice is:
A regular, 3-dimensional arrangement of alternating positive and negative ions.
Each ion is surrounded by oppositely charged ions on all sides.
Held together by STRONG ELECTROSTATIC FORCES in all directions.
The word GIANT means there are enormous numbers of ions — a typical crystal contains billions of ions in a regular repeating pattern.
Example: Sodium chloride (NaCl):
Na⁺ ions and Cl⁻ ions alternate in all three dimensions.
Each Na⁺ is surrounded by 6 Cl⁻ ions (and vice versa).
The ratio of Na⁺ : Cl⁻ = 1:1 throughout the lattice.
This structure is very strong — requiring a lot of energy to break the many ion-ion attractions.
Properties of Ionic Compounds
The giant ionic lattice structure explains ALL the key properties:
HIGH MELTING AND BOILING POINTS:
The electrostatic forces between ions throughout the lattice are very strong.
A lot of energy is needed to overcome them and separate the ions.
NaCl melts at 801°C; MgO melts at 2852°C.
BRITTLE:
When force is applied, layers of ions shift slightly.
Similarly charged ions end up next to each other → repel strongly → the crystal SHATTERS rather than bending.
DO NOT CONDUCT ELECTRICITY WHEN SOLID:
Ions in the solid lattice are in fixed positions — they cannot move.
Electrical conductivity requires charged particles that can move freely.
DO CONDUCT ELECTRICITY WHEN MOLTEN OR IN SOLUTION:
When melted or dissolved in water, ions are FREE TO MOVE.
The mobile ions carry charge through the liquid → conducts electricity.
This is why ionic compounds are electrolytes.
Solubility of Ionic Compounds
Many ionic compounds are SOLUBLE IN WATER:
Water molecules (which are polar — they have a partial charge) are attracted to the ions.
Water molecules surround the ions and pull them away from the lattice one by one.
The ions become surrounded by water molecules (hydrated) — this is DISSOLVING.
When dissolved, the ions are spread throughout the solution and can move freely → the solution conducts electricity.
Not all ionic compounds are soluble:
Some lattices have very strong attractions (e.g. BaSO₄) that water cannot overcome → insoluble.
Insoluble ionic compounds can form as PRECIPITATES when two solutions are mixed.
Example: mixing silver nitrate with sodium chloride:
Ag⁺ ions + Cl⁻ ions → AgCl precipitate (a white solid that doesn't dissolve).
⚠️ Common Mistake
Ionic compounds conduct electricity when MOLTEN or in SOLUTION — NOT when solid. The ions in solid ionic compounds are held in fixed positions in the lattice and cannot move. Melting or dissolving gives the ions freedom to move — and moving charged particles = electrical conductivity.
📌 Key Note
Giant ionic lattice: regular 3D arrangement of alternating + and − ions held by strong electrostatic forces in all directions. High MP/BP (strong forces). Brittle (layers shift → ion repulsion). Solid: no conduction (ions fixed). Molten/dissolved: conducts (ions free to move).