Symbols, formulae and equations

Science Double Award 1370 · CCEA GCSE · 9 min read · Updated: 31 Aug 2026

Welcome to Symbols, Formulae, and Equations!

Welcome to one of the most fundamental chapters in GCSE Chemistry! Think of chemical symbols and equations as the language of chemistry. Just like we use letters to make words and words to make sentences, chemists use element symbols to write formulae, and formulae to write chemical equations.

Don't worry if balancing equations or working out formulae feels tricky at first. We are going to break everything down into easy, bite-sized steps with simple rules and helpful tricks!

1. Chemical Symbols: The Alphabet of Chemistry

Every element in the Periodic Table has its own unique chemical symbol. A symbol always consists of either one capital letter or one capital letter followed by a lowercase letter.

For example:
• Carbon = \( \text{C} \)
• Oxygen = \( \text{O} \)
• Sodium = \( \text{Na} \) (from its Latin name, natrium)
• Chlorine = \( \text{Cl} \)

Common Mistake to Avoid: Always write the second letter in lowercase! For instance, \( \text{Co} \) is the element cobalt, but \( \text{CO} \) is carbon monoxide (a compound containing carbon and oxygen).

Diatomic Molecules

Most elements exist as individual atoms, but there are seven non-metal elements that exist naturally in pairs. We call these diatomic molecules. When writing them as pure elements, they must always have a small subscript \( 2 \).

The 7 diatomic elements are:
• Hydrogen: \( \text{H}_2 \)
• Nitrogen: \( \text{N}_2 \)
• Oxygen: \( \text{O}_2 \)
• Fluorine: \( \text{F}_2 \)
• Chlorine: \( \text{Cl}_2 \)
• Bromine: \( \text{Br}_2 \)
• Iodine: \( \text{I}_2 \)

Memory Trick: Remember the name "HOFBrINCl" (pronounced Hof-brinkle) or the phrase "I Bring Clay For Our New House" (\( \text{I}_2, \text{Br}_2, \text{Cl}_2, \text{F}_2, \text{O}_2, \text{N}_2, \text{H}_2 \))!

Key Takeaway: Single elements get their standard symbol from the Periodic Table, but the seven diatomic elements always travel in pairs with a small subscript 2 when alone: \( \text{X}_2 \).

2. Writing Chemical Formulae

A chemical formula shows the types and numbers of atoms present in a substance. For example, water has the formula \( \text{H}_2\text{O} \), which tells us there are \( 2 \) hydrogen atoms bonded to \( 1 \) oxygen atom.

Valency and the "Swap and Drop" Method

Valency is the combining power of an element (how many bonds an atom can make or how many electrons it wants to lose, gain, or share). We can determine the valency of main group elements directly from their group in the Periodic Table:

• Group 1: Valency = \( 1 \) (Forms \( +1 \) ions)
• Group 2: Valency = \( 2 \) (Forms \( +2 \) ions)
• Group 3: Valency = \( 3 \) (Forms \( +3 \) ions)
• Group 4: Valency = \( 4 \)
• Group 5: Valency = \( 3 \) (Forms \( -3 \) ions)
• Group 6: Valency = \( 2 \) (Forms \( -2 \) ions)
• Group 7: Valency = \( 1 \) (Forms \( -1 \) ions)
• Group 0 / 8: Valency = \( 0 \) (Noble gases do not normally form bonds)

Step-by-Step: The "Swap and Drop" Method

To work out the formula of a compound made of two elements:
1. Write down the symbols of the elements.
2. Write their valencies underneath.
3. Swap the numbers over and drop them down to become subscripts.
4. Simplify the ratio if possible (e.g., \( 2:2 \) becomes \( 1:1 \)), and remember we do not write the number \( 1 \).

Example 1: Magnesium oxide
• Symbols: \( \text{Mg} \) and \( \text{O} \)
• Valencies: \( 2 \) and \( 2 \)
• Swap and simplify: \( 2:2 \) simplifies to \( 1:1 \)
• Formula: \( \text{MgO} \)

Example 2: Aluminium chloride
• Symbols: \( \text{Al} \) and \( \text{Cl} \)
• Valencies: \( 3 \) and \( 1 \)
• Swap and drop: The \( 3 \) goes to \( \text{Cl} \), and the \( 1 \) goes to \( \text{Al} \)
• Formula: \( \text{AlCl}_3 \)

Molecular (Polyatomic) Ions

Some ions are made of groups of atoms joined together that carry an overall charge. You must memorize these essential polyatomic ions for your CCEA exam:

• Hydroxide: \( \text{OH}^- \) (Valency = \( 1 \))
• Nitrate: \( \text{NO}_3^- \) (Valency = \( 1 \))
• Sulfate: \( \text{SO}_4^{2-} \) (Valency = \( 2 \))
• Carbonate: \( \text{CO}_3^{2-} \) (Valency = \( 2 \))
• Ammonium: \( \text{NH}_4^+ \) (Valency = \( 1 \))

Rule for Brackets: If you need more than one polyatomic ion in a formula, put the ion inside brackets and write the number outside at the bottom right.

Example 3: Magnesium hydroxide
• Symbols/Ions: \( \text{Mg} \) (valency \( 2 \)) and \( \text{OH} \) (valency \( 1 \))
• Swap and drop: \( \text{Mg} \) gets \( 1 \), \( \text{OH} \) gets \( 2 \)
• Formula: \( \text{Mg(OH)}_2 \) (Note: Writing \( \text{MgOH}_2 \) is incorrect because that would mean only 2 hydrogens and 1 oxygen!)

Example 4: Ammonium sulfate
• Ions: \( \text{NH}_4 \) (valency \( 1 \)) and \( \text{SO}_4 \) (valency \( 2 \))
• Swap and drop: \( \text{NH}_4 \) gets \( 2 \), \( \text{SO}_4 \) gets \( 1 \)
• Formula: \( \text{(NH}_4\text{)}_2\text{SO}_4 \)

Key Takeaway: Use the valency rules and the "Swap and Drop" method. Always use brackets around polyatomic ions if there is more than one of them.

3. Word Equations and State Symbols

A chemical reaction changes reactants into products:

\( \text{Reactants} \rightarrow \text{Products} \)

The arrow (\( \rightarrow \)) means "react to form" or "produces". Never use an equals sign (\( = \)) in a chemical equation!

State Symbols

State symbols show the physical state of each substance at room temperature. They are written in brackets after each formula:

• Solid: \( (s) \) — e.g., metals, precipitates, powders: \( \text{Mg}(s) \)
• Liquid: \( (l) \) — pure liquids: \( \text{H}_2\text{O}(l) \)
• Gas: \( (g) \) — gases: \( \text{O}_2(g) \), \( \text{CO}_2(g) \)
• Aqueous solution: \( (aq) \) — dissolved in water: \( \text{NaCl}(aq) \), \( \text{HCl}(aq) \)

Did You Know? Acids are always aqueous solutions \( (aq) \) because their acidic properties only appear when dissolved in water!

Key Takeaway: State symbols tell us whether a substance is a solid, liquid, gas, or dissolved in water.

4. Balancing Symbol Equations

The Law of Conservation of Mass states that no atoms can be created or destroyed in a chemical reaction. Therefore, the total number of atoms of each element on the left side (reactants) must equal the total number on the right side (products).

Golden Rules for Balancing Equations:

1. NEVER change the small subscript numbers inside a chemical formula (e.g., do not change \( \text{O}_2 \) to \( \text{O}_3 \)).
2. ONLY add big numbers in front of the formulae (coefficients), e.g., \( 2\text{H}_2\text{O} \).
3. A big number multiplies every atom in that formula. For example, \( 2\text{Al}_2\text{O}_3 \) contains \( 2 \times 2 = 4 \) aluminium atoms and \( 2 \times 3 = 6 \) oxygen atoms.

Step-by-Step Balancing Example

Let's balance the reaction between magnesium and oxygen gas to produce magnesium oxide.

Step 1: Write the unbalanced equation
\( \text{Mg} + \text{O}_2 \rightarrow \text{MgO} \)

Step 2: Count the atoms on both sides
• Left (Reactants): \( 1 \times \text{Mg} \), \( 2 \times \text{O} \)
• Right (Products): \( 1 \times \text{Mg} \), \( 1 \times \text{O} \)
Oxygen is unbalanced!

Step 3: Add a big number in front to balance one element
Put a \( 2 \) in front of \( \text{MgO} \) to balance oxygen:
\( \text{Mg} + \text{O}_2 \rightarrow 2\text{MgO} \)

Step 4: Recount the atoms
• Left: \( 1 \times \text{Mg} \), \( 2 \times \text{O} \)
• Right: \( 2 \times \text{Mg} \), \( 2 \times \text{O} \)
Now Magnesium is unbalanced!

Step 5: Balance the remaining element and add state symbols
Put a \( 2 \) in front of \( \text{Mg} \) on the left:
\( 2\text{Mg}(s) + \text{O}_2(g) \rightarrow 2\text{MgO}(s) \)

Both sides have \( 2 \times \text{Mg} \) and \( 2 \times \text{O} \). The equation is now completely balanced!

Key Takeaway: Balancing is a game of tallying atoms on both sides and using big multiplier numbers in front of formulas until both sides match.

5. Ionic Equations

When ionic substances dissolve in water, their ions separate and move freely. In many reactions (such as neutralisation or precipitation), only some of these ions actually take part in the reaction. The ions that do not change during the reaction are called spectator ions.

An ionic equation shows only the reacting particles, leaving out the spectator ions.

How to Write an Ionic Equation

Example: The reaction between hydrochloric acid and sodium hydroxide

Step 1: Write the full balanced symbol equation with state symbols
\( \text{HCl}(aq) + \text{NaOH}(aq) \rightarrow \text{NaCl}(aq) + \text{H}_2\text{O}(l) \)

Step 2: Split all aqueous \( (aq) \) ionic compounds into their separate ions
(Leave solids \( (s) \), liquids \( (l) \), and gases \( (g) \) exactly as they are!)
\( \text{H}^+(aq) + \text{Cl}^-(aq) + \text{Na}^+(aq) + \text{OH}^-(aq) \rightarrow \text{Na}^+(aq) + \text{Cl}^-(aq) + \text{H}_2\text{O}(l) \)

Step 3: Identify and cross out the spectator ions
Notice that \( \text{Na}^+(aq) \) and \( \text{Cl}^-(aq) \) appear on both sides completely unchanged. These are spectator ions.

Step 4: Write out the remaining ionic equation
\( \text{H}^+(aq) + \text{OH}^-(aq) \rightarrow \text{H}_2\text{O}(l) \)

This is the classic ionic equation for any acid-alkali neutralisation reaction!

Precipitation Reactions Example

When silver nitrate solution reacts with sodium chloride solution, a white solid precipitate of silver chloride forms:
Full equation: \( \text{AgNO}_3(aq) + \text{NaCl}(aq) \rightarrow \text{AgCl}(s) + \text{NaNO}_3(aq) \)
Ionic equation: \( \text{Ag}^+(aq) + \text{Cl}^-(aq) \rightarrow \text{AgCl}(s) \)

Key Takeaway: Ionic equations zoom in on the real chemical change by crossing out spectator ions that remain dissolved in solution.

Quick Review Summary Checklist

Before sitting your exam, make sure you can:
• Identify and write the formulas for the 7 diatomic elements (\( \text{H}_2, \text{N}_2, \text{O}_2, \text{F}_2, \text{Cl}_2, \text{Br}_2, \text{I}_2 \)).
• Use group numbers to find the valency of elements and use "Swap and Drop" to write correct chemical formulae.
• Recall the formulas of molecular ions: \( \text{OH}^- \), \( \text{NO}_3^- \), \( \text{SO}_4^{2-} \), \( \text{CO}_3^{2-} \), \( \text{NH}_4^+ \).
• Balance chemical equations by placing whole numbers in front of formulae.
• Correctly include state symbols: \( (s) \), \( (l) \), \( (g) \), and \( (aq) \).
• Construct ionic equations by removing spectator ions.

Quick check

Can you answer these now?

Open each question to check the key ideas from this chapter.

What are the rules for writing chemical symbols for elements?

A chemical symbol consists of either one capital letter (e.g. \( \text{C} \)) or one capital letter followed by a lowercase letter (e.g. \( \text{Na} \), \( \text{Cl} \)).

What is a diatomic molecule and which seven elements exist naturally as diatomic molecules?

A diatomic molecule consists of two atoms bonded together. The seven diatomic elements are \( \text{H}_2 \), \( \text{N}_2 \), \( \text{O}_2 \), \( \text{F}_2 \), \( \text{Cl}_2 \), \( \text{Br}_2 \), and \( \text{I}_2 \).

How is the valency of a main group element determined from the Periodic Table?

Valency corresponds to group number: Group 1 = 1, Group 2 = 2, Group 3 = 3, Group 4 = 4, Group 5 = 3, Group 6 = 2, Group 7 = 1, and Group 0/8 = 0.

How do you use the "Swap and Drop" method to write chemical formulae for binary compounds?

Write down the symbols, write their valencies underneath, swap the numbers over, drop them as subscripts, and simplify the ratio if possible (omitting the number 1).

What are the chemical formulae and valencies of the five common polyatomic ions you must know?

Hydroxide: \( \text{OH}^- \) (1), Nitrate: \( \text{NO}_3^- \) (1), Sulfate: \( \text{SO}_4^{2-} \) (2), Carbonate: \( \text{CO}_3^{2-} \) (2), and Ammonium: \( \text{NH}_4^+ \) (1).

What are the four state symbols used in chemical equations and what do they mean?

\( (s) \) = solid, \( (l) \) = pure liquid, \( (g) \) = gas, and \( (aq) \) = aqueous solution (dissolved in water).

Why must chemical equations be balanced, and what are the main rules for balancing them?

Mass is conserved so atom counts must match on both sides; you can only balance by adding large coefficients in front of formulae, never by changing subscript numbers.

What is an ionic equation and what are spectator ions?

An ionic equation shows only the reacting species; spectator ions are ions present in the mixture that do not change during the reaction and are omitted from the equation.

What is the standard ionic equation for any acid-alkali neutralisation reaction?

\( \text{H}^+(aq) + \text{OH}^-(aq) \rightarrow \text{H}_2\text{O}(l) \)

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