Symbols, formulae and equations - (CCEA)

• Each element is represented by a chemical symbol. These symbols can be joined together with numbers to make chemical formulae.

• Chemical formulae can be worked out using a ‘swap and drop’ method that uses valency numbers for different atoms and ions.

• Chemical equations show how reactants interact to make products in a chemical reaction. All chemical symbol equations must be balanced.

• There are two other types of chemical equation: ionic equations and half equations (higher tier only).

Chemical symbols

Each element in the periodic table is represented by its own chemical symbol.

A chemical symbol has one or two letters, and it must always start with a capital letter.

Any second letter must always be lower case.

For example, Mg is the correct symbol for magnesium, but mg, mG and MG are wrong.

Take care to write chemical symbols correctly.

Diatomic elements

The formulae for most elements is just their symbol.

The exceptions are some non-metal elements known as diatomic elements .

They contain two atoms of the same element covalently bonded together.

Key fact

You will need to memorise the seven diatomic elements.

A mnemonic that can help is “Now I Only Have Bright and Clever Friends”.

A formula gives information about the type and the number of each atom present in a compound.

The formula for sodium sulfate is Na₂SO₄. It tells you that sodium sulfate contains:

• two sodium atoms

• one sulfur atom and

• four oxygen atoms

The subscript numbers refer to the element before them.

Key fact

If there is just one atom in the formula a ‘1’ is not written.

Sometimes a formula includes brackets.

The atoms inside the brackets are multiplied by the number outside the brackets.

Aluminium sulfate’s formula is Al₂(SO₄)₃.

It contains:

• 2 atoms of aluminium
• 3 atoms of sulfur
• 12 atoms of oxygen

A compound is two or more elements that are chemically combined.

Some compounds are regarded as ‘common’ and their formulae need to be memorised, while for other compounds you will need to work out the formulae yourself.

What are common compounds?

The table below shows some common compounds.

Key fact

You will need to memorise this list of common compounds and their formulae.

It is possible to work out the formula of an ionic compound from its name.

Ionic compounds contain positive and negative ions.

You need to be able to work out how many of each ion are required for the formula.

Valency numbers are important for working out formulae.

The valency of an ion tells you how many bonds it is able to form or how many electrons it will lose or gain.

There is a pattern that links the group number of an element to its valency.

The ‘swap and drop’ method is useful for working out formulae:

1. Write down the symbols of both elements in the compound.

2. Look up the group number of the elements and write the valency number to the top right of each element.

3. Swap the two valency numbers over, and drop them down to the bottom of each symbol.

4. Write the finished formula.

Key fact

If both valency numbers are the same, then the numbers cancel out and there is just one of each element in the compound.

A molecular ion is a charged particle containing more than one element.

The rules for carrying out a swap and drop in a compound containing a molecular ion are the same as for single elements.

The valency of the molecular ion is simply the size of the charge on the ion.

In cases where a number greater than one is swapped across to a molecular ion, brackets are required to go around the formula of the molecular ion.

In a chemical reaction, the substances that react together are called reactants and the substances that are made are called products.

Word equations

All reactions can be represented in word equations, which must contain an arrow (→)

reactants → products

For example, magnesium can react with oxygen to produce magnesium oxide:

Magnesium + oxygen → magnesium oxide

Key fact

Chemical equations contain an arrow and not an equals sign.

The arrow means 'reacts to make'.

No atoms are created or destroyed in a chemical reaction – they are just rearranged.

Balanced symbol equations show the original arrangements and the new ones.

• Each element must have equal numbers of atoms on each side of the equation.

• You are not allowed to change any formulae to balance an equation (e.g. you cannot change MgO into MgO₂).

• You can add a ‘big’ number in front of a formula to multiply all the atoms in that formula by that number.

A method that can help with balancing equations is the ‘bubble method.’

Click through the arrows below to see how this method works.

Chemical equations use state symbols:

Ionic compounds react when they are dissolved.

However, not all the ions react – some spectator ions do not react and remain unchanged.

Spectator ions are not shown in an ionic equation.

There are two different types of ionic equations:

• Neutralisation
• Displacement

When an acid reacts with an alkali, the acid’s hydrogen ions react with the alkali’s hydroxide ions to produce water.

The other ions are left unchanged.

For example, when hydrochloric acid (HCl) is mixed with potassium hydroxide (KOH), the hydrogen ions (H⁺) react with the hydroxide ions (OH^-).

The chloride (Cl^-) and potassium (K⁺) ions are spectator ions.

Ionic equation for neutralisation, including state symbols: H⁺(aq) + OH^-(aq) → H₂O(l)

The ionic equation for neutralisation is always the same for any neutralisation reaction.

It is worth learning it off by heart.

In displacement reactions, the more reactive metal displaces (takes the place of) a less reactive metal from a metal compound.

For example, when magnesium (Mg) reacts with copper(II) sulfate (CuSO₄), the magnesium atoms react with the copper(II) ions in the copper(II) sulfate.

The sulfate ions are spectator ions and do not take part in the reaction.

Final ionic equation, including state symbols: Mg(s) + Cu²⁺(aq) → Cu(s) + Mg²⁺(aq)

A half equation contains only a single element.

It shows the change of an atom/molecule into an ion, or vice versa.

The process for writing a balanced ionic equation is as follows:

1. Write down the reactant and the product.
2. Balance the atoms.
3. Write the total charge underneath each species in the equation.
4. Balance the charge by adding electrons.

Write the half equation for the conversion of chlorine molecules into chloride ions.

Answer = Cl₂ + 2e^- → 2Cl^-