Chemguide: Support for CIE A level Chemistry

Learning outcome 9.2

The Periodic Table

Periodicity of chemical properties of the elements in Period 3

Learning outcome 9.2.7

This statement asks you to deduce the bonding in an oxide or chloride from its chemical or physical properties. It doesn't say that these are restricted to the oxides or chlorides of Period 3 elements, and so it is safer not to assume that.

Before you go on, you should find and read the statement in your copy of the syllabus.

Physical properties

This is no different from deducing the bonding in any other sort of compound from its physical properties.

If a substance is a high melting point solid, then it will be a giant structure of some kind - either giant ionic or giant covalent (CIE use the term "giant molecular"). To decide whether it is giant ionic (such as NaCl or MgO) or giant covalent (such as SiO2) needs more information.

For example:  If you have a high melting point compound which doesn't undergo electrolysis when it is molten (assuming you can melt it!), then it can't contain ions, and so must be giant covalent.

If a substance is a gas, or liquid, or low melting point solid, then it will be a simple molecule with covalent bonding.

Chemical properties

Effect of water on chlorides

If a chloride just dissolves in water to give a simple solution with a pH close to 7, then it is ionic. Most, but not all, ionic chlorides are soluble in water.

Note:  But care! As the chloride dissolves in water, the metal ions become hydrated. Water molecules attach to them. As the charge on the positive ion increases, these ions become more and more acidic. If you are following this section of the syllabus through in order, you will already have seen this with regards to the hydrated magnesium and aluminium ions.

If a chloride reacts with water to produce an acidic solution, and steamy fumes of hydrogen chloride gas, then it is covalent. Most, but not all covalent chlorides behave in this way. The only simple one I can think of off-hand which doesn't is CCl4.

Effect of water on oxides

If an oxide reacts with water to give an acidic solution, then it is covalent. (There are exceptions if you go beyond Period 3. For example, there are a few neutral covalent oxides like CO and H2O.)

If an oxide reacts with water to give an alkaline solution, then it is ionic. However, not all ionic oxides will react with water.

Effect of acids or bases on oxides

Ionic oxides tend to be basic; covalent oxides tend to be acidic.

If an oxide reacts with a base to form a salt, then obviously it is an acidic oxide, and must be covalent.

If an oxide reacts with an acid to form a salt, then it is a basic oxide, and must be ionic.

Obviously, if an oxide reacts with both an acid and a base, then it must be amphoteric - such as aluminium oxide. Amphoteric oxides will tend to be ionic, but with a fair amount of covalent character.


Whether you have a chloride or an oxide, if the molten compound undergoes electrolysis, then it must contain ions. If it doesn't undergo electrolysis, then it is covalent.

A final comment

A lot of this isn't very clear-cut, and it isn't difficult to find exceptions. It is important that you look to see what sort of questions the examiners are asking by looking at recent question papers and mark schemes. You should be doing that for every topic you study, anyway, but it is particularly essential in this messy periodicity section.

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© Jim Clark 2020