Chemical Formulae

Reminder: Elements, compounds and mixtures

An element is made up from only one type of atom for example sodium or sulfur.

When two or more elements are chemically combined a compound is formed, for example sodium sulfide.

When two or more substances are mixed together but not joined we get a mixture.

When compounds contain only two elements the compound name ends in -ide for example potassium oxide.

When a compound contains 3 or more elements joined together and one of the elements is oxygen the compound name ends with -ate or ite. For example potassium nitrate.

Compounds ending in the word hydroxide are an exception and contain at least two elements including oxygen and hydrogen. For example potassium hydroxide contains potassium, oxygen and hydrogen.

Writing Chemical Formula

Each element has a symbol. Compounds can also be represented using these symbols.

The formulae for compounds include element symbols and numbers. The numbers tell us the ratio of atoms in the compound.

For example:

In CO₂, the ratio of carbon to oxygen atoms is 1:2.

In MgO, the ratio of magnesium to oxygen atoms is 1:1.

In Na₂CO₃, the ratio of sodium to carbon to oxygen atoms is 2:1:3

In order to work out the formula of a compound you need to use the valency of the elements in the compound.

The valency tells you how many links, or bonds each atom can make to other atoms. The valency of an element is related to how many electrons are in the outer shell.

You will need to be able to state the valency of an element based on its group in the Periodic Table. These are listed below (click on the title to display the groups and valencies)

The noble gases have a valency of 0 as they do not usually combine with other elements.

To work out the formula of a compound you must use the symbols and usually the valency.

The exception to this is when there is a prefix (for example di- or tri-) in the compound’s name and this will be explained later.

Formulae of compounds made from two elements

When writing these formulae the steps outline below are used.

This is sometimes described as the SVSDF method – Symbols, Valency, Swap, Divide, Formula.

Step 1 - Write down the Symbols of both the elements involved.

Step 2 - Beneath each symbol, write its Valency. Memorising the valency table is useful.*

Step 3 - Swap the valencies over.

Step 4 - If the valencies can be simplified, Divide them both by the smaller of the two numbers. If one of the numbers is already one, then they cannot be divided and simplified any further.

Step 5 - Write the Formula.

*If the elements both have the same valency then Steps 3 and 4 can be left out and the formula will simply be the two symbols. For example sodium and chlorine have both got a valency of one and therefore the formula is just NaCl.

Some examples of two element compound formulae

Notice: the number 1 is not written in this or in any other chemical formula. The formula is just MgCl₂ and not Mg₁Cl₂

In this next example unlike the one above, after the valencies have been swapped over the numbers should be simplifies (as they can both be divided by two). This was not needed in example 1.

Two element compounds containing a transition metal

The valency of transition metals cannot be worked out from its position on the Periodic Table. The valency of the transition metals is always indicated by Roman numerals in the compound name. For example copper(II) chloride indicates that copper has a valency of two in this compound.

The steps to work out the formula of a two element compound containing transition metals are the same as shown previously (SVSDF).

Using copper(II) chloride as an example:

Compounds with a prefix in the name

When a compound has a prefix in the name e.g. sulfur dioxide do NOT use the valency cross-over method (SVSDF).

The name of the compound describes the formula you do not need to use the valency to work out the formula! You must know the main prefixes used in chemical formulae and these are listed in the table below.

For example:

The formula for carbon dioxide is CO₂ and the formula for sulfur trioxide is SO₃. If there is no prefix before one of the element names in the compound it means there is only one of that element in the formula. When there are prefixes in a compound name you do not need to simplify /divide the formula.

Compounds with more than two elements

If a compound name ends in ite or ate it contains 3 or more elements one of which is oxygen.

Hydroxides also contain oxygen, hydrogen and one other element.

For formulae of compounds with 3 elements you must use the table of "Formulae of Selected Ions containing more than one kind of Atom" on page 8 of the data booklet.

The charge for these ions (column heading) is the same as the valency of the ions. For example phosphate has a valency of three.

If the valency for both parts of the formula is equal you do not need to swap them over. You can simply write down the formulae for example sodium hydroxide’s formula is just NaOH.

When the valencies of the two chemicals are different we use the same method as before (SVSDF), however there are some extra things (stated below) to remember.

The formula of these ions cannot be changed for example sulfate is always SO₄ and carbonate is CO₃. The four and three in these formulae represent the number of oxygen atoms and are not the valency.

It is also important to know that if you cross over the valencies of the chemicals and you need to multiple one of the ions by a number (other than one) you will need to put that part of the formula in brackets with the number outside the brackets after the formula and in subscript. This is shown in the example on the next page. Apart from these specific rules the formulae can be calculated using the SVSDF method.

Notice that OH must be in brackets with a 2 outside and not as OH₂ because OH₂ means two hydrogen atoms and one oxygen, (OH)₂ represents two hydroxides (two hydrogen and two oxygen atoms).

Another example: ammonium sulfide

Notice that the number 4 does not get swapped over as this is not the valency. Ammonium has a valency of one and is found on page 8 of the data booklet.

Writing formulae summary

To work out chemical formulae:

• Use the valency cross over rule (SVSDF) except where the compound name contains a prefix such as mono- or di-

• Transition metal valencies are shown by Roman numerals after the metal name for example in copper(II) sulfate copper has a valency of 2.

• In compounds containing more than two elements you need to use page 8 in the data booklet and use the size of the ion charge as the valency. For example phosphate PO₄ has a valency of 3 (because the column heading states “three negative”).

Next section: Diatomic elements and compounds