Formulae
Section: 3. Stoichiometry | Syllabus: Cambridge IGCSE Chemistry (0620)
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.
Common Mistakes
- ❌ Thinking the subscript applies to all elements before it
- ✓ a subscript only applies to the element or bracket immediately before it - in H₂SO₄, the 4 only applies to oxygen, not sulfur or hydrogen
- ❌ Confusing CO and CO₂
- ✓ CO is carbon monoxide (1 C, 1 O), a toxic gas; CO₂ is carbon dioxide (1 C, 2 O), what we breathe out - the subscript makes a huge difference
- ❌ Thinking (aq) means the substance is water
- ✓ (aq) means "aqueous" - dissolved in water - not that the substance itself is water; NaCl(aq) is salt dissolved in water, not water itself
- ❌ Thinking all elements exist as diatomic molecules
- ✓ only seven elements are diatomic (H₂, N₂, F₂, O₂, I₂, Cl₂, Br₂) - elements like carbon, sodium, and iron exist as single atoms or giant structures
Interactive revision notes, videos and practice questions load below.