Writing Chemical Formulae

atom.gif (1130 bytes) Writing Chemical Formulae

A particle made up of two or more atoms combined together is called a MOLECULE. Molecules may be made up of atoms of the same element such as Hydrogen gas (H2 - two atoms of Hydrogen), or atoms of different elements such as Sodium Chloride (NaCl). Substances made up of molecules consisting of atoms of different elements are called COMPOUNDS. (The word "compound" means made up of several parts.)

NOTE: Some molecules are very large and may consist of hundreds, or even thousands of atoms.

The FORMULA of a substance is a way of describing the composition of the molecules of the substance. For example H2 means the smallest unit of Hydrogen gas consists of two atoms of Hydrogen combined together.

NaCl means the smallest unit of Sodium Chloride consists of one atom of Sodium combined with one atom of Chlorine. It could be written as: Na1Cl1, but it is a convention that the "1s" are never written.

NOTE: Remember that a charged atom is called an "ion". It would be more correct to say that the smallest unit of Sodium Chloride consists of one ion of Sodium and one ion of Chlorine.

To work out the formula of a compound you need to know the valency of each of its ions ...

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In the previous sections we looked at how many electrons from each atom were exchanged in order to provide a stable outer shell. Remember that Magnesium Chloride was composed of one ion of Magnesium and two ions of Chlorine ...

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The formula of Magnesium Chloride is therefore: MgCl2

When working out the formula of an ionic compound you must ask yourself: "How many of each ion are needed so that the number of pluses equals the number of minuses?" ie the number of electrons donated equals the number of electrons accepted.

For example ... The valency of Calcium is . The valency of Bromine is . To work out the formula of Calcium Bromide we must ask ourselves: "How many of each ion are needed so that the number of pluses equals the number of minuses?" The answer is two ions of Bromine and one ion of Calcium. This will give us two pluses and two minuses. The formula is therefore CaBr2

NOTE1: When working out the pluses and minuses, the smallest possible combination is used. You might have said that you needed two ions of Calcium and four ions of Bromine giving four pluses and four minuses. In a way you are correct, but the formula is not Ca2Br4

NOTE2: When writing the formula of an ionic compound the positive ion is always written first. It is not considered correct to write Br2Ca even though it gives us the same information.

NOTE3: To be able to work out the formula of an ionic compound you MUST know the valencies from the Valency Table above. There are a few more from the 'middle' of the Table that you should also know such as Iron, Copper and Mercury. You must learn them 'parrot fashion'. If you do not you will never succeed in Chemistry. Start learning your valencies now!!

Once you know the valency of the ions in a compound you can use a 'short-cut' method to work out the formula.This involves 'cross multiplying' the charges on each ion.

As outlined above, the 'ones' are never written so the formula becomes: CaBr2

Consider Aluminium Oxide ...

Using the first approach - ask yourself: "How many of each ion are needed so that the number of pluses equals the number of minuses?" This time it is not quite so simple. The answer is three Oxygen ions and two Aluminium ions give six pluses and six minuses. Using the 'cross-multiply' short-cut:

The formula for Aluminium Oxide becomes: Al2O3 which means that a molecule of Aluminium Oxide consists of two ions of Aluminium combined with three ions of Oxygen.

NOTE: The 'cross-multiply short cut can sometimes be a bit confusing. You need to keep in mind that you are trying to 'balance' the number of electrons donated with the number accepted. In the case of a compound such as Calcium Oxide where one ion is 2+ and the other is 2- the formula is CaO, NOT Ca2O2.

To complete the QUIZ for this section you can refer to the Valency Table above, but in future you must know the valencies of the common ions. Learn them!! You will not succeed in Chemistry if you don't learn them now!!

Writing Chemical Formulae - QUIZ

Your Name and School -

Do you Certify the answers below to be all your own work? -

1. What is the difference between an "element" and a "compound"? Give an example of each. -

2. An imaginary compound has a formula of X3Y2Z6. What does this tell us?-

3. The imaginary compound "Exalite" has a formula of Ex4Lt3. How many electrons are in the outer shell of each element? -

4. What are the formulae of the following compounds ... Calcium Oxide, Lithium Bromide, Sodium Oxide? -
(NOTE: Just write your answer in the form: "X2Y3". Don't worry about trying to make the numbers 'subscript' for this QUIZ.)

5. Write the names of the compounds represented by the following formulae: Na20, BaCl2, KF. -

6. What are the valencies of Carbon in (a) Carbon Dioxide (b) Carbon Monoxide? What feature about the outer shell of Carbon may help explain the strange behaviour of this ion-

7. Explain why CaXe is unlikely to exist? -

8. What is the URL of a Web page containing information about Chemical Formulae (apart from this one)?

Feedback, comments, questions -

(Make sure your NAME and SCHOOL are included.)

This form will be emailed to a 'marker'. You will receive feedback shortly. Satisfactory performance on this and other Quizzes is required for the granting of a "Completion Certificate" for this Course.