Formulae
Section: Chemistry | Syllabus: Cambridge IGCSE Combined Science (0653)
What is a Chemical Formula?
A chemical formula is a representation of a substance using chemical symbols and numbers, showing which elements are present and how many atoms of each are in one unit of the substance.
- Uses chemical symbols from the periodic table
- A subscript number shows how many atoms of the element immediately before it; no subscript means 1 atom
- Example: H₂O means 2 hydrogen atoms and 1 oxygen atom; CO₂ means 1 carbon atom and 2 oxygen atoms
Molecular Formula
The molecular formula shows the actual number of atoms of each element present in one molecule of a substance.
- H₂O - water (2 hydrogen, 1 oxygen)
- C₆H₁₂O₆ - glucose (6 carbon, 12 hydrogen, 6 oxygen)
- H₂SO₄ - sulfuric acid (2 hydrogen, 1 sulfur, 4 oxygen)
The empirical formula shows the simplest whole-number ratio of atoms instead of the actual number - covered in detail, with calculations, later in this section.
Reading Formulae with Brackets
Brackets group a set of atoms that is repeated. The subscript after the closing bracket multiplies everything inside it.
- Ca(OH)₂ = 1 Ca + 2 × (1 O + 1 H) = 1 Ca, 2 O, 2 H (not 1 O and 1 H!)
- Al₂(SO₄)₃ = 2 Al + 3 × (1 S + 4 O) = 2 Al, 3 S, 12 O
Worked Example: Counting Atoms
- Question: how many atoms in total are in one formula unit of Mg(NO₃)₂? [2]
- Answer: Mg = 1; (NO₃)₂ = 2 × (1 N + 3 O) = 2 N + 6 O; total = 1 Mg + 2 N + 6 O = 9 atoms [2]
Deducing a Formula from a Model or Diagram
Exam questions sometimes show a ball-and-stick model, a dot-and-cross diagram, or a labelled diagram of a structure, and ask you to deduce the formula by counting the atoms or ions shown.
- Count each type of atom or ion shown in the diagram
- Write the count as a subscript after the correct symbol
- For ionic structures shown as a repeating grid, count the ratio of each ion type shown, not the total number of spheres in the whole diagram
Count each atom or ion type shown in a model or diagram to deduce the formula
Worked Example: Formula from a Diagram
- Question: a diagram shows a molecule with 1 central nitrogen atom bonded to 3 hydrogen atoms. State the molecular formula. [1]
- Answer: NH₃ [1]
State Symbols
State symbols are letters written in brackets after a formula to show the physical state of a substance.
| Symbol | State | Example |
|---|---|---|
| (s) | Solid | NaCl(s) |
| (l) | Liquid | H₂O(l) |
| (g) | Gas | CO₂(g) |
| (aq) | Aqueous (dissolved in water) | NaCl(aq) |
Common Element Symbols
| Symbol | Element | Symbol | Element |
|---|---|---|---|
| H | Hydrogen | O | Oxygen |
| C | Carbon | N | Nitrogen |
| Na | Sodium | K | Potassium |
| Mg | Magnesium | Ca | Calcium |
| Cl | Chlorine | Br | Bromine |
| Fe | Iron | Cu | Copper |
| Zn | Zinc | Al | Aluminium |
| S | Sulfur | P | Phosphorus |
Some symbols don't match the English name because they come from Latin names: Na (sodium, natrium), K (potassium, kalium), Fe (iron, ferrum), Cu (copper, cuprum).
Diatomic Molecules
Seven elements exist naturally as diatomic molecules - two atoms of the same element bonded together.
| Element | Formula | State at Room Temperature |
|---|---|---|
| Hydrogen | H₂ | Gas |
| Nitrogen | N₂ | Gas |
| Fluorine | F₂ | Gas |
| Oxygen | O₂ | Gas |
| Iodine | I₂ | Solid |
| Chlorine | Cl₂ | Gas |
| Bromine | Br₂ | Liquid |
When writing equations, always use the diatomic form (e.g. O₂, not O) for these seven elements in their elemental state.
Word and Symbol Equations
A chemical equation represents a reaction, showing the reactants (starting materials, on the left) reacting to form the products (on the right), separated by an arrow (→, meaning "produces" or "forms").
- Word equation: uses the names of substances, e.g. magnesium + oxygen → magnesium oxide
- Symbol equation: uses chemical formulae, e.g. Mg + O₂ → MgO
Converting Word to Symbol Equations
- Write the word equation
- Replace each name with its correct chemical formula
- Balance the equation (see below)
- Add state symbols if required
Balancing Chemical Equations
A balanced equation has the same number of atoms of each element on both sides, following the law of conservation of mass - atoms cannot be created or destroyed in a chemical reaction, only rearranged.
- Only change the large numbers in front of formulae (coefficients)
- Never change a subscript within a formula - that would turn it into a different substance (H₂O₂ is hydrogen peroxide, not water!)
- Balance one element at a time; leave hydrogen and oxygen until last, since they often appear in several formulae
- Remember the seven diatomic elements exist as pairs, not single atoms
A balanced equation has exactly the same number of each type of atom on both sides
Worked Example: Balancing Step by Step
- Question: balance the equation for the combustion of methane: CH₄ + O₂ → CO₂ + H₂O [2]
- Answer: balance H first by putting 2 in front of H₂O: CH₄ + O₂ → CO₂ + 2H₂O; now O is unbalanced (2 left, 4 right), so put 2 in front of O₂: CH₄ + 2O₂ → CO₂ + 2H₂O; check - C: 1=1, H: 4=4, O: 4=4 ✓ [2]
Equations with State Symbols
State symbols show the physical state of each reactant and product.
- 2Mg(s) + O₂(g) → 2MgO(s)
- CaCO₃(s) → CaO(s) + CO₂(g)
- HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)
Worked Example: Deducing a Symbol Equation with State Symbols
Interactive revision notes, videos and practice questions load below.