Structure and Bonding - Ionic Compound Properties

Giant Ionic Lattices

Ionic compounds are made of positive and negative ions that arrange themselves into a regular three-dimensional pattern called a giant ionic lattice. Think of it like a massive 3D jigsaw puzzle where each piece (ion) is surrounded by oppositely charged neighbours. Strong electrostatic forces of attraction act in every direction, holding the structure together. These forces (ionic bonds) extend throughout the entire crystal.

• Each lattice has no overall charge - it is electrically neutral

• The repeating pattern can be described by a unit cell - the smallest section that shows the complete structure

  • In sodium chloride, the unit cell contains Na+Na+ and Cl−Cl− ions in a 1:11:1 ratio

  • Other ionic crystals follow the same principle, even when the charge ratios are different

Because the bonding extends throughout the entire structure rather than being limited to small groups of atoms, we call the lattice giant rather than simple molecular.

Melting and Boiling Points

To melt or boil an ionic compound, you must overcome the strong electrostatic forces of attraction between the ions in the giant lattice. A large amount of thermal energy is needed to do this.

This is why ionic compounds have characteristically high melting points and boiling points. They are solid at room temperature because not enough energy is available to break down the strong lattice structure.

Electrical Conductivity in Different States

Solid state

In a solid lattice, ions are locked in fixed positions and can only vibrate slightly. Solid ionic compounds do not conduct electricity because the charge carriers (ions) cannot move through the crystal.

Molten (liquid) state

When an ionic solid is heated beyond its melting point, the rigid lattice structure breaks down. The ions become free to move around in the liquid, so molten ionic compounds conduct electricity.

Dissolved in water (aqueous solutions)

Water molecules are polar - they have a slightly positive and slightly negative end. These polar water molecules pull ions away from the lattice, creating hydrated ions that can move freely in solution. Therefore, solutions of ionic compounds also conduct electricity.

When electricity flows through the solution:

1. Cations (positive ions) move towards the negative electrode (cathode)

2. Anions (negative ions) move towards the positive electrode (anode)

Because actual charged particles are moving, we can use electrolysis to break down the compound, although the products depend on which ions are present and how reactive they are.

Comparison table

Linking Structure to Properties

We can explain the large-scale properties of ionic compounds by understanding their microscopic structure:

• Giant lattice → strong bonding → high melting and boiling points

• Fixed ions in solid → no electrical conduction

• Mobile ions in liquid/solution → electrical conduction

These principles apply to all ionic substances, not just sodium chloride.

Key terms

Giant ionic lattice - A continuous three-dimensional arrangement of oppositely charged ions held together by ionic bonds.

Electrostatic attraction - The force of attraction between particles with opposite electric charges.

Cation - A positively charged ion that has lost one or more electrons.

Anion - A negatively charged ion that has gained one or more electrons.

Electrical conductivity - A measure of how easily electric charge can flow through a material.

Electrolysis - The breakdown of a compound using electricity when the compound is molten or dissolved.

Worked example

Question: Sodium chloride must be heated to 801°C before it melts, but it does not conduct electricity at room temperature. Explain both observations using your knowledge of ionic structure.

Solution:

1. High melting point: The Na+Na+ and Cl−Cl− ions in solid NaCl are held together by strong electrostatic forces in a giant lattice structure. A very high temperature (801°C) is needed to provide enough energy to overcome these strong forces and break apart the lattice.

2. No conduction at room temperature: In the solid state, the ions are locked in fixed positions within the lattice and cannot move. Since electrical conduction requires moving charged particles, solid NaCl cannot conduct electricity.

Real-world application: De-icing roads

When calcium chloride (CaCl2CaCl2​) is dissolved in water to de-ice roads, the Ca2+Ca2+ and Cl−Cl− ions become free to move in solution. This solution can conduct electricity.

If we carried out electrolysis on this solution:

• Ca2+Ca2+ ions would move to the negative electrode, but because calcium is very reactive, water gets reduced instead, producing hydrogen gas

• Cl−Cl− ions would move to the positive electrode and could be oxidised to chlorine gas (in concentrated solutions)

The key point is that dissolved ions can move and carry charge, enabling electrical conduction.

Investigating Electrical Conductivity of Ionic Compounds

Aim: To demonstrate that ionic compounds only conduct electricity when their ions are free to move.

Apparatus:

• Power pack and conductivity tester (bulb and electrodes)

• Connecting leads

• Samples of solid sodium chloride, distilled water

• Bunsen burner, crucible, and tongs

• Eye protection

Method:

1. Test the conductivity of solid NaCl at room temperature - record whether the bulb lights up

2. Dissolve some NaCl in distilled water and test the conductivity of the solution

3. Teacher demonstration: Carefully heat NaCl in a crucible until molten, then test conductivity

Safety:

• Wear eye protection throughout

• Only teachers should handle molten salt using tongs

• Disconnect power supply before changing samples

• Work in a fume cupboard when heating ionic compounds

Expected Results:

• Solid NaCl: No conduction (bulb doesn't light)

• NaCl solution: Conducts electricity (bulb lights)

• Molten NaCl: Conducts electricity (bulb lights)

Conclusion: Ionic compounds only conduct electricity when their ions are free to move (when molten or dissolved in water).

Comparison table