DECIDING WHAT TYPE OF STRUCTURE A SUBSTANCE HAS

This page explains how you can decide what sort of structure a substance has by looking at its physical properties. It starts with a very simple look at solids, liquids and gases.

Changes of state as a guide to forces between particles

The arrangements of particles in solids, liquids and gases

A simple view of the arrangement of the particles in solids, liquids and gases looks like this:

Solids

In the solid, the particles are touching, and the only motion allowed to them is vibration. The particles may be arranged regularly (in which case, the solid is crystalline), or at random (giving waxy solids like candles or some forms of polythene, for example).

The particles are held in the solid by forces which depend on the actual substance - ionic bonds, covalent bonds, metallic bonds, hydrogen bonds or van der Waals attractions.

Melting and freezing

If energy is supplied by heating the solid, the heat energy causes greater vibrations until the particles eventually loosen from each other to form a liquid. The heat energy required to convert 1 mole of solid into a liquid at its melting point is called the enthalpy of fusion.

When a liquid freezes, the reverse happens. At some temperature, the motion of the particles is slow enough for the forces of attraction to be able to hold the particles as a solid. As the new bonds are formed, heat energy is evolved.

Liquids

In a liquid, the particles are mainly touching, but some gaps have appeared in the structure. These gaps allow the particles to move, and so the particles are arranged randomly. Unless melting has broken up a substance consisting only of covalent bonds (a giant covalent structure), the forces that held the solid particles together are also present in the liquid, but in a somewhat loosened form.

Boiling and condensing

If more heat energy is supplied, the particles eventually move fast enough to break all the attractions between them, and the liquid boils. The heat energy required to convert 1 mole of liquid into a gas at its boiling point is called the enthalpy of vaporisation.

If the gas is cooled, at some temperature the gas particles will slow down enough for the attractions to become effective enough to condense it back into a liquid. Again, as those forces are re-established, heat energy is released.

Gases

In a gas, the particles are entirely free to move. At ordinary pressures, the distance between individual particles is of the order of ten times the diameter of the particles. At that distance, any attractions between the particles are fairly negligible.

Deducing the type of bonding from physical properties

The physical state and other properties

The best place to start is usually the physical state.

Melting point isn't always a good guide to the size of the attractions between particles, because the attractive forces have only been loosened on melting - not broken entirely. Boiling point is a much better guide, because enough heat has now been supplied to break the attractive forces completely. The stronger the attractions, the higher the boiling point.

That being said, melting points are often used to judge the size of attractive forces between particles in solids, but you will find the occasional oddity. Those oddities usually disappear if you consider boiling points instead.

So . . . If it is a gas, liquid or low melting point solid, it will consist of covalently bound molecules (except the noble gases which have molecules consisting of single atoms).

The size of the melting point or boiling point gives a guide to the strength of the intermolecular forces. If it is also soluble in water (without reacting), that suggests a small molecule capable of hydrogen bonding - or, at least, a small very polar molecule.

If it is a high melting point solid, it will be a giant structure - either ionic, metallic or giant covalent.

Solubility in water (without reaction) suggests it's ionic. If the substance also undergoes electrolysis when it is molten, that would confirm that it was ionic.