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O Level · Chemistry 0620

Formulae & Relative Masses

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Formulae

Molecular formulae
Diatomic elements
Reading diagrams

Ionic Compounds

Empirical formulae
Ion charges
Swap-and-drop method

Equations

Word equations
Symbol equations
Balancing

Relative Masses

Atomic mass (Ar)
Molecular mass (Mr)
Reacting masses

1. Formulae — How We Write What Atoms and Molecules Are Made Of

What Is a Molecular Formula?

A molecular formula is a shorthand way of showing what atoms a molecule is made of, and how many of each. It tells you the actual number of each element present.

For example:

H₂ means two hydrogen atoms bonded together
H₂O means two hydrogen atoms and one oxygen atom
CH₄ (methane) means one carbon atom and four hydrogen atoms

The numbers below the line (subscripts) tell you exactly how many atoms there are. If there's no number, it means there's one atom.

The Seven Diatomic Elements — A Very Important List

You must memorise these seven elements. They always exist as pairs of atoms bonded together, and they always have a subscript 2.

Element	Name	Formula	What It Looks Like
H	Hydrogen	H₂	Gas (colourless)
N	Nitrogen	N₂	Gas (in air)
O	Oxygen	O₂	Gas (we breathe it)
F	Fluorine	F₂	Gas (pale yellow)
Cl	Chlorine	Cl₂	Gas (yellow-green)
Br	Bromine	Br₂	Liquid (brown-red)
I	Iodine	I₂	Solid (black)

Memory trick: HNOFClBrI — think "H Not For Clever Bromine Iodine" or just memorise the periodic table positions.

Deducing Molecular Formulae from Diagrams

Exam questions will often show you a coloured ball-and-stick diagram and ask you to write the formula. Here's how:

Count each colour separately. Assign each colour to an element (a key is always given).
Write the symbols in order. Usually carbon first (if present), then hydrogen, then others.
Add subscripts for multiples. If there are 4 hydrogen atoms, write H₄.

Worked Example: Deducing a Formula from a Diagram

You are given a diagram showing: 4 black spheres, 10 white spheres, arranged as a chain.

The key states: black = carbon, white = hydrogen

Step 1: Count the atoms
Black (carbon) = 4
White (hydrogen) = 10

Step 2: Write the formula
Carbon first, then hydrogen: C₄H₁₀

Answer: The molecular formula is C₄H₁₀ (butane)

Practice Question 1

2. Empirical Formulae & Formulae of Ionic Compounds

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Also in the full note

3. Writing Equations — Word Equations and Symbol Equations
4. Relative Atomic and Molecular Masses
What to Memorise — Quick Reference
Exam Tips — What Examiners Look For (And What They Don't!)
Final Summary: Bringing It All Together
What Is an Empirical Formula?
Ionic Compounds — The Swap-and-Drop Method
The Swap-and-Drop Method Explained

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Formulae & Relative Masses

Formulae

Ionic Compounds

Equations

Relative Masses

1. Formulae — How We Write What Atoms and Molecules Are Made Of

What Is a Molecular Formula?

The Seven Diatomic Elements — A Very Important List

Deducing Molecular Formulae from Diagrams

Worked Example: Deducing a Formula from a Diagram

Practice Question 1

2. Empirical Formulae & Formulae of Ionic Compounds

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