Symbols, formulae and equations

Chemistry 1110 · CCEA GCSE · 10 min read · Updated: 31 Aug 2026

Welcome to Symbols, Formulae and Equations

Welcome to one of the most important foundation topics in CCEA GCSE Chemistry! Just like learning music requires understanding musical notes, chemistry has its own universal language. Chemical symbols, formulae, and equations allow scientists anywhere in the world to describe exactly what substances are made of and how they react.

Don't worry if writing formulae or balancing equations seems a bit tricky at first. We will break every concept down into clear, step-by-step pieces with simple rules and helpful tricks to get you scoring full marks in your exams.

1. Chemical Symbols and Diatomic Elements

Rules for Writing Element Symbols

Every chemical element is represented by a unique chemical symbol:

• Single-letter symbols: Always written as one capital letter (e.g., Carbon is \( \text{C} \), Hydrogen is \( \text{H} \), Oxygen is \( \text{O} \), and Nitrogen is \( \text{N} \)).
• Two-letter symbols: Always written with the first letter capitalised and the second letter in lower-case (e.g., Sodium is \( \text{Na} \), Calcium is \( \text{Ca} \), Copper is \( \text{Cu} \), and Chlorine is \( \text{Cl} \)).

Examiner Warning: Take care with capital letters! Writing \( \text{NA} \) or \( \text{CL} \) is marked incorrect in exams. Also, be careful not to confuse the element Cobalt, \( \text{Co} \), with the compound carbon monoxide, \( \text{CO} \).

Diatomic Elements

Most non-metal elements exist as individual atoms, but there are seven specific elements that exist naturally as diatomic molecules. This means two identical atoms are covalently bonded together.

Whenever you write these elements on their own in a chemical equation, they must always have a subscript \( 2 \):

• 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 phrase "I Bring Clay From Our New House" (\( \text{I}_2, \text{Br}_2, \text{Cl}_2, \text{F}_2, \text{O}_2, \text{N}_2, \text{H}_2 \)) or the word "HOFBrINCl" (pronounced Hoff-brinkle) to recall all seven instantly!

Key Takeaway: Elements have a one- or two-letter symbol (always Capital then lower-case). The seven diatomic elements always travel in pairs when uncombined: \( \text{H}_2, \text{N}_2, \text{O}_2, \text{F}_2, \text{Cl}_2, \text{Br}_2, \text{I}_2 \).

2. Interpreting and Deducing Chemical Formulae

How to Count Atoms in a Formula

A chemical formula tells you the exact ratio of atoms or ions in a substance. Subscript numbers apply only to the element immediately before them. When brackets are present, multiply everything inside the brackets by the subscript outside.

Example 1: Water, \( \text{H}_2\text{O} \)
• \( 2 \) Hydrogen atoms
• \( 1 \) Oxygen atom
• Total = \( 3 \) atoms

Example 2: Magnesium Nitrate, \( \text{Mg(NO}_3\text{)}_2 \)
• Magnesium: \( 1 \) atom
• Nitrogen: \( 1 \times 2 = 2 \) atoms
• Oxygen: \( 3 \times 2 = 6 \) atoms
• Total = \( 1 + 2 + 6 = 9 \) atoms

Writing Formulae for Ionic Compounds

Ionic compounds are made of positive ions (cations) and negative ions (anions). In every stable compound, the total positive charge must equal the total negative charge so that the overall compound has no charge (it is neutral).

Important Polyatomic (Compound) Ions to Memorise

CCEA requires you to know the names and charges of these specific compound ions:

• Hydroxide: \( \text{OH}^- \) (charge: \( -1 \))
• Nitrate: \( \text{NO}_3^- \) (charge: \( -1 \))
• Hydrogencarbonate: \( \text{HCO}_3^- \) (charge: \( -1 \))
• Sulfate: \( \text{SO}_4^{2-} \) (charge: \( -2 \))
• Carbonate: \( \text{CO}_3^{2-} \) (charge: \( -2 \))
• Dichromate: \( \text{Cr}_2\text{O}_7^{2-} \) (charge: \( -2 \))
• Ammonium: \( \text{NH}_4^+ \) (charge: \( +1 \))

Transition Metals and Roman Numerals

Transition metals can form ions with different charges. The Roman numeral in brackets tells you the positive charge on the metal ion:

• Iron(II) = \( \text{Fe}^{2+} \)
• Iron(III) = \( \text{Fe}^{3+} \)
• Copper(II) = \( \text{Cu}^{2+} \)

Step-by-Step: The "Swap and Drop" (Valency) Method

Step 1: Write down the symbols and charges of the two ions.
Step 2: Cancel out the \( + \) and \( - \) signs and keep just the numbers (valencies).
Step 3: Swap the numbers diagonally to become the subscript for the other ion.
Step 4: Simplify the ratio if possible (e.g., \( 2:2 \) becomes \( 1:1 \)). If a compound ion needs a number greater than \( 1 \), put brackets around it!

Worked Example A: Calcium Hydroxide
• Ions: \( \text{Ca}^{2+} \) and \( \text{OH}^- \)
• Charges / Valencies: \( \text{Ca} = 2 \), \( \text{OH} = 1 \)
• Swap the numbers: \( \text{Ca}_1\text{(OH)}_2 \)
• Final Formula: \( \text{Ca(OH)}_2 \)

Worked Example B: Ammonium Sulfate
• Ions: \( \text{NH}_4^+ \) and \( \text{SO}_4^{2-} \)
• Charges / Valencies: \( \text{NH}_4 = 1 \), \( \text{SO}_4 = 2 \)
• Swap the numbers: \( (\text{NH}_4)_2(\text{SO}_4)_1 \)
• Final Formula: \( (\text{NH}_4)_2\text{SO}_4 \)

Common Mistake to Avoid: Never write \( \text{MgOH}_2 \) or \( \text{AlSO}_{4\,3} \). You must use brackets around polyatomic ions when you need more than one: \( \text{Mg(OH)}_2 \) and \( \text{Al}_2\text{(SO}_4\text{)}_3 \).

Key Takeaway: Ionic compounds are neutral. Use ion charges and valencies to balance charges, and enclose compound ions in brackets whenever more than one is required.

3. Reactants, Products and Conservation of Mass

Reactants and Products

• Reactants: The starting substances that react together (written on the left side of an equation).
• Products: The new substances formed by the reaction (written on the right side of an equation).

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

The Law of Conservation of Mass

The Law of Conservation of Mass states that in any chemical reaction, no atoms are created or destroyed. The atoms in the reactants are simply rearranged to form new products.

Because atoms are neither gained nor lost:

Total mass of reactants = Total mass of products

Real-World Analogy: Imagine building a toy house using 50 Lego blocks. If you break it apart and build a car using all the pieces, the car still contains exactly 50 blocks and weighs the same as the house.

Key Takeaway: Mass is always conserved in chemical reactions because no atoms are made or lost; they are simply rearranged.

4. Word Equations and Balanced Symbol Equations

Word Equations

A word equation uses the full chemical names of substances to describe a reaction. Always use a standard arrow (\( \rightarrow \)) pointing to the products, never an equals sign (\( = \)).

Example:
\( \text{magnesium} + \text{hydrochloric acid} \rightarrow \text{magnesium chloride} + \text{hydrogen} \)

State Symbols

State symbols show the physical state of each substance at room temperature or reaction conditions:

• \( \text{(s)} \) = Solid (e.g., metals, precipitates, powders)
• \( \text{(l)} \) = Pure liquid (e.g., pure liquid water \( \text{H}_2\text{O(l)} \))
• \( \text{(g)} \) = Gas (e.g., \( \text{O}_2\text{(g)} \), \( \text{CO}_2\text{(g)} \))
• \( \text{(aq)} \) = Aqueous solution (a substance dissolved in water, e.g., acids like \( \text{HCl(aq)} \) or dissolved salts like \( \text{NaCl(aq)} \))

Examiner Warning: Do not write \( \text{(l)} \) for acids or dissolved salts. Acids and solutions are always \( \text{(aq)} \)!

How to Balance Symbol Equations

A balanced symbol equation must have the exact same number of each type of atom on both sides of the arrow.

The Golden Rule of Balancing:

NEVER alter the little subscript numbers inside a chemical formula! You can only balance an equation by placing whole numbers (coefficients) in front of the entire chemical formula.

Step-by-Step Balancing Guide:
1. Write the correct formula for every reactant and product.
2. Count the number of atoms of each element on the reactant side and the product side.
3. Pick an element that is not balanced and place a whole number in front of the formula containing it.
4. Recount and repeat until all atoms match on both sides.

Worked Example: Reaction of Hydrogen with Oxygen to form Water
• Unbalanced equation: \( \text{H}_2 + \text{O}_2 \rightarrow \text{H}_2\text{O} \)
• Count atoms: Reactants have \( 2\text{ H}, 2\text{ O} \); Products have \( 2\text{ H}, 1\text{ O} \).
• Balance Oxygen by placing a \( 2 \) in front of \( \text{H}_2\text{O} \):
\( \text{H}_2 + \text{O}_2 \rightarrow 2\text{H}_2\text{O} \)
• Recount: Reactants have \( 2\text{ H} \); Products now have \( 4\text{ H} \).
• Balance Hydrogen by placing a \( 2 \) in front of \( \text{H}_2 \):
\( 2\text{H}_2 + \text{O}_2 \rightarrow 2\text{H}_2\text{O} \)
• Final balanced check: \( 4\text{ H} \) and \( 2\text{ O} \) on both sides. Fully balanced!

Key Takeaway: Balance symbol equations using large whole numbers in front of formulas. Ensure the correct state symbol is applied: \( \text{(s)} \), \( \text{(l)} \), \( \text{(g)} \), or \( \text{(aq)} \).

5. Ionic Equations and Spectator Ions

What is an Ionic Equation?

In many reactions that happen in aqueous solution (such as neutralisation and precipitation reactions), not all ions take part in the reaction. Some ions float in the water before the reaction and remain unchanged in the water afterwards.

These unchanged ions are called spectator ions because they just "watch" without getting involved, like spectators at a football match.

An ionic equation simplifies the reaction by showing only the species that change state or charge during the reaction, leaving out the spectator ions.

Step-by-Step: Writing an Ionic Equation for Neutralisation

Let's look at 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 soluble ionic compounds (aqueous substances) into their separate ions.
(Note: Leave solids, liquids, and gases as complete molecules/compounds!)
\( \text{H}^+\text{(aq)} + \text{Cl}^-\text{(aq)} + \text{Na}^+\text{(aq)} + \text{OH}^-\text{(aq)} \rightarrow \text{Na}^+\text{(aq)} + \text{Cl}^-\text{(aq)} + \text{H}_2\text{O(l)} \)

Step 3: Identify and cancel the spectator ions that appear identical on both sides.
• \( \text{Na}^+\text{(aq)} \) appears unchanged on both sides \( \rightarrow \) Cancel out!
• \( \text{Cl}^-\text{(aq)} \) appears unchanged on both sides \( \rightarrow \) Cancel out!

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

This simple ionic equation represents the core of all acid-alkali neutralisation reactions in GCSE Chemistry!

Key Takeaway: Ionic equations show only the reacting particles. Identify aqueous ions that remain unchanged on both sides and cancel them out as spectator ions.

Quick Review Checklist

Before sitting your exam, make sure you can confidently do each of the following:

• Write element symbols with correct capitalisation (e.g., \( \text{Na} \), \( \text{Cl} \), \( \text{Ca} \)).
• Recall the seven diatomic elements: \( \text{H}_2, \text{N}_2, \text{O}_2, \text{F}_2, \text{Cl}_2, \text{Br}_2, \text{I}_2 \).
• Count the total number of atoms in a formula with brackets, such as \( \text{Mg(NO}_3\text{)}_2 \).
• State the charges for key compound ions: \( \text{OH}^- \), \( \text{NO}_3^- \), \( \text{HCO}_3^- \), \( \text{SO}_4^{2-} \), \( \text{CO}_3^{2-} \), \( \text{Cr}_2\text{O}_7^{2-} \), \( \text{NH}_4^+ \).
• Use valencies and brackets to write balanced ionic formulae (e.g., \( \text{Ca(OH)}_2 \)).
• State the Law of Conservation of Mass.
• Balance chemical equations by adding coefficients in front of formulae.
• Apply the four state symbols: \( \text{(s)} \), \( \text{(l)} \), \( \text{(g)} \), and \( \text{(aq)} \).
• Cancel spectator ions to construct a net ionic equation.

Quick check

Can you answer these now?

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

What are the rules for writing single-letter and two-letter chemical symbols for elements?

Single-letter symbols are written as one capital letter, while two-letter symbols have the first letter capitalised and the second letter in lower-case.

Which seven elements naturally exist as diatomic molecules when uncombined?

Hydrogen (\( \text{H}_2 \)), Nitrogen (\( \text{N}_2 \)), Oxygen (\( \text{O}_2 \)), Fluorine (\( \text{F}_2 \)), Chlorine (\( \text{Cl}_2 \)), Bromine (\( \text{Br}_2 \)), and Iodine (\( \text{I}_2 \)).

How do you count the total number of atoms in a chemical formula containing brackets?

Multiply the number of each atom inside the brackets by the subscript number outside the brackets, then add any atoms outside the brackets.

What are the formulae and charges for the key polyatomic (compound) ions required in GCSE Chemistry?

\( \text{OH}^- \), \( \text{NO}_3^- \), \( \text{HCO}_3^- \) (charge \( -1 \)); \( \text{SO}_4^{2-} \), \( \text{CO}_3^{2-} \), \( \text{Cr}_2\text{O}_7^{2-} \) (charge \( -2 \)); and \( \text{NH}_4^+ \) (charge \( +1 \)).

What does a Roman numeral indicate in the name of a transition metal compound?

It indicates the positive charge (valency) on the transition metal ion.

How is the "swap and drop" method used to write ionic formulae?

Take the numerical values of the ion charges (valencies), swap them diagonally to become subscripts for the opposite ions, simplify the ratio, and use brackets if more than one compound ion is needed.

What does the Law of Conservation of Mass state regarding chemical reactions?

No atoms are created or destroyed in a chemical reaction; they are simply rearranged, so the total mass of reactants equals the total mass of products.

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

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

What is the golden rule for balancing chemical symbol equations?

Never change the subscript numbers within a chemical formula; only balance by placing whole number coefficients in front of formulae.

What are spectator ions and what does a net ionic equation represent?

Spectator ions are ions that remain unchanged in state and charge during a reaction; a net ionic equation shows only the species that actively participate and change.

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