Ionic compounds: Bonds, Structure & Properties

When a metal reacts with a non-metal, electrons transfer from the metal to the non-metal. This makes positive ions (cations) and negative ions (anions). The ionic bond is the force that holds these ions together.

The exact, mark-earning definition is: an ionic bond is a strong electrostatic force of attraction between oppositely charged ions. "Electrostatic" simply means between electric charges — opposite charges attract.

A key idea examiners look for: this attraction acts in all directions at once. A positive ion is not bonded to just one negative ion — it is pulled towards every oppositely charged ion around it. That is why ionic substances do not form simple molecules; they build huge structures (see the next section).

• Opposite charges attract: Na⁺ is attracted to Cl⁻.
• Like charges repel: Na⁺ pushes away other Na⁺ ions — this becomes important for brittleness.
• The bond is strong — overcoming it takes a lot of energy.

Watch your wording. "An ionic bond is when electrons are shared" is wrong (that is covalent bonding). In ionic bonding electrons are transferred, then the resulting ions are held by electrostatic attraction.

A formula like NaCl makes it look as if one Na⁺ pairs up with one Cl⁻ to make a tiny "molecule". That is not what happens. Because each ion attracts oppositely charged ions in all directions, the ions pack into a giant ionic lattice — a regular, repeating 3D arrangement of billions of alternating positive and negative ions.

In sodium chloride, each Na⁺ ion is surrounded by 6 Cl⁻ neighbours, and each Cl⁻ is surrounded by 6 Na⁺ neighbours. This pattern repeats over and over in all three dimensions.

Why "giant"? There is no fixed number of ions in the structure — the lattice keeps repeating for as long as the crystal lasts. That is why we use the word giant rather than "molecule". The chemical formula (NaCl) just gives the simplest ratio of ions (1 Na⁺ : 1 Cl⁻), not the number of ions present.

Why it matters. Every property in the next two sections comes back to this picture: many strong electrostatic attractions, in all directions, holding the whole lattice together.

1. High melting and boiling points. To melt an ionic compound you have to break apart the lattice — which means overcoming all the strong electrostatic forces of attraction between the oppositely charged ions, in every direction. Because there are so many of these strong forces, a large amount of energy is needed. So ionic compounds are usually solids at room temperature with high melting and boiling points (for example, sodium chloride melts at 801 °C).

2. Hard but brittle. The lattice is held together so tightly that the solid resists scratching — it is hard. But ionic crystals are also brittle: a sharp blow makes them shatter rather than bend.

The reason is in the lattice. When a force pushes one layer of ions so that it slides over the next:

• ions of like charge (e.g. + above +) end up next to each other;
• like charges repel strongly;
• this repulsion pushes the layers apart, so the crystal splits / shatters along that plane.

So "hard but brittle" is not a contradiction: the strong electrostatic forces make it hard, but the fact that those forces depend on a fixed arrangement of charges makes it brittle — disturb the arrangement and the same forces turn into repulsion.

For a substance to conduct electricity it needs charged particles that are free to move and carry the charge. In ionic compounds the charge carriers are the ions themselves (not electrons — electrons are for metals). Whether they can move depends on the state.

State	Conducts?	Reason
Solid	NO	The ions are held in fixed positions in the lattice and cannot move to carry charge.
Molten (liquid)	YES	Heating overcomes the electrostatic forces; the lattice breaks down and the ions are free to move.
In solution (aqueous)	YES	Water breaks up the lattice; the ions become free to move through the solution.

The single most important phrase to memorise: ionic compounds conduct only when the ions are free to move (molten or dissolved). As a solid, the ions are fixed in the lattice and cannot move — so no current flows.

When they conduct, the ions actually move towards the electrodes — positive ions to the negative electrode, negative ions to the positive electrode — carrying the charge through the liquid.