Course 7.3Lesson 2 of 3

Properties of ionic compounds

An ionic compound is a crystal lattice, a regular pattern of positive and negative ions in which each ion is surrounded by ions of the opposite kind. Ionic compounds are hard but brittle solids with high melting and boiling points, because the attraction between the ions is strong. Most dissolve in water and not in oils. They do not conduct electricity as solids, because the ions cannot move, but they conduct when melted or dissolved, when the ions are free. Their current is carried by ions and not by electrons.

about 25 min1 simulation2 practice questions

Lesson 2 of 3Show all lessons

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A salt crystal under a lens

Look at grains of common salt through a magnifying glass. Each grain is a tiny cube. The shape of the cube is not an accident. It shows how the particles inside are arranged.

The crystal

An ionic compound is not made of separate molecules. It is a regular pattern of positive and negative ions, in which every ion is surrounded by ions of the opposite kind. This pattern is a crystal lattice.

A cube of sodium chloride crystal drawn as a grid of balls. Purple balls are sodium ions and green balls are chloride ions. Each ion has neighbours of the opposite kind in all directions.

In sodium chloride, each sodium ion is surrounded by six chloride ions, and each chloride ion is surrounded by six sodium ions. There is no single "molecule of NaCl". The formula NaCl only says that the ions are in the ratio $1 : 1$ .

The properties of ionic compounds

Physical state. They are solids at room temperature. The strong attraction between the ions holds them in the pattern. They are hard, but brittle. If we hit the crystal, the layers of ions shift by a step, and then ions of the same charge come face to face. They repel each other, and the crystal breaks into pieces. A metal behaves in a different way. Its layers can slide, so a metal bends and does not break.

High melting and boiling points. A lot of energy is needed to pull the ions apart. So ionic compounds melt at a high temperature.

Compound	Melting point
Sodium chloride	$1074\ \text{K}$
Magnesium oxide	$3125\ \text{K}$
Calcium chloride	$1045\ \text{K}$
Sugar (not ionic)	$459\ \text{K}$
Wax (not ionic)	about $330\ \text{K}$

Magnesium oxide has the highest melting point of the list. Its ions have a charge of $2+$ and $2-$ , and the attraction between charges of this size is stronger than the attraction between $1+$ and $1-$ . This is why magnesium oxide is used to line the inside of furnaces.

Solubility. Most ionic compounds dissolve in water, because the water molecules pull the ions apart and surround them. They are generally insoluble in kerosene, petrol and other oils.

Conduction of electricity.

In the solid, the ions are held in place, and they cannot move. The solid does not conduct.
When melted, the ions are free to move, and the liquid conducts.
When dissolved in water, the ions are free, and the solution conducts.

So the current in an ionic compound is carried by ions, not by electrons as in a metal. This is why the extraction of reactive metals uses molten ionic compounds, as you see in the next lesson.

A test you can do

Set up a battery, a bulb and two electrodes. Put the electrodes in a dry crystal of salt. The bulb does not light. Now put the electrodes in salt dissolved in water. The bulb lights. The same salt gave two different results, because in the solution the ions can move.

You should find that heat alone does nothing until the salt melts, and that water lights the bulb at once.

How to recognise an ionic compound

A substance that is a hard, brittle solid, with a high melting point, that dissolves in water and conducts when it is dissolved or melted, is very likely ionic.

Property	Ionic compound	Molecular (covalent) compound
State	solid	solid, liquid or gas
Melting point	high	low
Dissolves in water	usually yes	often no
Conducts when solid	no	no
Conducts when melted	yes	no

Practise with new numbers

Try it in Python

This program lists the melting points of some compounds, and gives them in degrees celsius.

Common mistakes

Saying that a solid ionic compound has no ions. It has ions, but they cannot move.
Saying that the current is carried by electrons. In an ionic compound, ions carry the current.
Thinking that "hard" means "not brittle". A crystal of salt is hard, and it still breaks when hit.
Thinking that every compound with a metal is ionic. Most are, but check the properties.

Remember these

An ionic compound is a crystal lattice of ions. It is hard and brittle, with a high melting point.
It dissolves in water. It does not conduct as a solid, and it conducts when melted or dissolved.
Metals conduct with electrons. Ionic compounds conduct with ions.