Chemical
Formulae
and Equations
Understanding Chemical
Composition and Reactions
Based on Chapter 4 of Chemistry for Cambridge IGCSE Coursebook
(5th Edition) by Richard Harwood, Ian Lodge, and Chris Millington
H
Hydrogen
Na
C
Carbon
Cl
Sodium
Chlorine
P
Fe
Phosphorus
Iron
N
Nitrogen
Ca
Calcium
Br
Bromine
O
Oxygen
S
Sulfur
Mg
Magnesium
Chemical Symbols
What are Chemical Symbols?
Chemical symbols are one- or two-letter abbreviations used to
represent the elements. They are the building blocks of all matter
and are essential for writing chemical formulae.
Rules for Writing Symbols
Common Elements
H
Hydrogen
C
Carbon
N
Nitrogen
O
Oxygen
Na
Sodium
Cl
Chlorine
Ca
Calcium
Fe
Iron
Pb
Lead
Rule 1: The first letter is always uppercase
Rule 2: If there is a second letter, it is always lowercase
Examples: H (hydrogen), He (helium), Ca (calcium)
Some elements have symbols that come from their Latin names, not
their English names. For example, Fe for iron (from "ferrum") and Pb
for lead (from "plumbum").
Writing Chemical Formulae
How to Write Formulae
A chemical formula shows the composition of a compound using
chemical symbols and subscript numbers.
Examples of Simple Compounds
H2O
Water: 2 hydrogen atoms + 1 oxygen atom
Rule 1: Write the symbol for each element in the compound
Rule 2: Add a subscript number after each symbol to show how many
atoms of that element are present
Rule 3: If there is only one atom of an element, no subscript number is
needed
For example, water contains 2 hydrogen atoms and 1 oxygen atom, so
its formula is written as H2O. The subscript "2" shows there are 2
CO2
Carbon dioxide: 1 carbon atom + 2 oxygen atoms
NH3
Ammonia: 1 nitrogen atom + 3 hydrogen atoms
hydrogen atoms.
CH4
Methane: 1 carbon atom + 4 hydrogen atoms
O2
Oxygen gas: 2 oxygen atoms (diatomic molecule)
Molecular Formulae
What is a Molecular Formula?
A molecular formula shows the number and type of different atoms in one molecule of a compound. It is written using chemical symbols with
subscript numbers to indicate how many atoms of each element are present.
The subscript number tells us how many atoms of that element are bonded together in the molecule. If there is no subscript number, it means there is
just one atom of that element.
H₂O
NH₃
Water
Ammonia
2 hydrogen atoms
1 oxygen atom
1 nitrogen atom
3 hydrogen atoms
CH₄
CO₂
Methane
Carbon Dioxide
1 carbon atom
4 hydrogen atoms
1 carbon atom
2 oxygen atoms
Key Point: The molecular formula tells us the exact composition of one molecule. For example, every molecule of water always contains exactly 2 hydrogen
atoms and 1 oxygen atom.
Empirical Formulae
The empirical formula shows the simplest whole number ratio of the different atoms in a compound. It is the most reduced form of a chemical
formula and may differ from the molecular formula.
Empirical Formula
Molecular Formula
Shows the simplest ratio of atoms in a compound
Shows the actual number of atoms in one molecule
May not represent the actual number of atoms in a molecule
Represents the true composition of the compound
Found by dividing all subscripts by their greatest common factor
Can be calculated from the empirical formula and molar mass
Example: C₂H₆ has an empirical formula of CH₃
Example: Ethane (C₂H₆) is the molecular formula
Empirical vs Molecular Formulae Examples
Worked Examples
Example 1: Hydrogen Peroxide
Molecular formula:
H₂O₂
Empirical formula:
HO (divide both subscripts by 2)
The ratio of H to O is 1:1, the simplest whole number ratio
Example 2: Glucose
Molecular formula:
C₆H₁₂O₆
Empirical formula:
CH₂O (divide all subscripts by 6)
The ratio of C:H:O is 1:2:1, the simplest whole number ratio
Example 3: Ethane
Molecular formula:
C₂H₆
Empirical formula:
CH₃ (divide both subscripts by 2)
Relative Atomic Mass (Ar)
Calculating Relative Molecular Mass
Relative molecular mass (Mr) is the sum of the relative atomic masses (Ar) of all the atoms in a molecule.
It is calculated by adding together the Ar values of all atoms present in the chemical formula, taking into account the number of each type of atom.
Mr = (number of atoms of element A × Ar of A) + (number of atoms of element B × Ar of B) + ...
Example 1: Water (H₂O)
Example 2: Carbon Dioxide (CO₂)
H₂O
CO₂
H: 2 × 1 = 2
C: 1 × 12 = 12
O: 1 × 16 = 16
O: 2 × 16 = 32
─────────────
─────────────
Mr = 18
Mr = 44
Relative Molecular Mass (Mr)
More Worked Examples
Writing Word Equations
A word equation is a way of describing a chemical reaction using the names of the substances involved, rather than their chemical formulae.
Word equations show what reacts (the reactants) and what is produced (the products). They are often the first step in writing a chemical equation
before using symbols and formulae.
General Format of a Word Equation
Reactants → Products
Reactants: The substances that react together (on the left side)
Arrow (→): Shows the direction of the reaction
Products: The new substances formed (on the right side)
Writing Symbol Equations
Balancing Chemical Equations
Balancing a chemical equation means adjusting the numbers in front of each formula so that there are equal numbers of each type of
atom on both sides of the equation.
This is required by the law of conservation of mass: atoms cannot be created or destroyed in a chemical reaction, only rearranged.
Method for Balancing Equations
• Count the number of atoms of each element on both sides
• Identify which elements are not balanced
• Add coefficients (numbers in front) to balance the atoms
• Check that all elements are now balanced
• Use the smallest whole number coefficients possible
Balancing Chemical Equations
Worked Examples
Example 1: Hydrogen + Oxygen
Example 2: Iron + Oxygen
H₂ + O₂ → H₂O
Fe + O₂ → Fe₂O₃
Unbalanced: 2 H on left, 2 H on right ✓; 2 O on left, 1 O on right ✗
Unbalanced: 1 Fe on left, 2 Fe on right; 2 O on left, 3 O on right
2H₂ + O₂ → 2H₂O
4Fe + 3O₂ → 2Fe₂O₃
Balanced: 4 H atoms and 2 O atoms on each side
Balanced: 4 Fe and 6 O atoms on each side
Example 3: Magnesium + Oxygen
Example 4: Methane Combustion
Mg + O₂ → MgO
CH₄ + O₂ → CO₂ + H₂O
Unbalanced: 1 Mg on left, 1 Mg on right ✓; 2 O on left, 1 O on right ✗
Unbalanced: 4 H on left, 2 H on right; 2 O on left, 4 O on right
2Mg + O₂ → 2MgO
CH₄ + 2O₂ → CO₂ + 2H₂O
Balanced: 2 Mg and 2 O atoms on each side
Balanced: 1 C, 4 H, and 4 O atoms on each side
State Symbols in Equations
State symbols are letters written in parentheses after each substance in a chemical equation to show the physical state (or condition) of that
substance.
They provide important information about whether reactants and products are solids, liquids, gases, or dissolved in solution.
(s)
(l)
Solid
Liquid
A substance with a fixed shape and volume. Particles are tightly packed and
vibrate in fixed positions.
A substance with a fixed volume but takes the shape of its container.
Particles can move freely but are still close together.
Examples: iron, copper, salt, carbon
Examples: water, ethanol, mercury
(g)
(aq)
Gas
Aqueous Solution
A substance with no fixed shape or volume. Particles are far apart and
move rapidly in all directions.
A substance dissolved in water. The substance is dispersed throughout the
water as individual ions or molecules.
Examples: oxygen, carbon dioxide, hydrogen
Examples: salt solution, acid solution, copper sulfate solution
State Symbols in Equations
Worked Examples
Complete Equations with State Symbols
Example 1: Combustion of Hydrogen
2H₂(g) + O₂(g) → 2H₂O(l)
Example 2: Thermal Decomposition of Calcium Carbonate
CaCO₃(s) → CaO(s) + CO₂(g)
Example 3: Reaction of Magnesium with Dilute Acid
Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)
Example 4: Neutralization Reaction
HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)
Ionic Compounds and Formulae
Ionic compounds are formed when electrons are transferred from one atom to another, creating ions (charged particles).
Ionic bonding occurs between a metal (which loses electrons to form positive ions) and a non-metal (which gains electrons to form negative ions). The
compound is electrically neutral overall.
How to Deduce Ionic Formulae
• Identify the metal ion and its charge (usually from the group number)
• Identify the non-metal ion and its charge
• Balance the charges so the total charge equals zero
• Write the formula with the metal first, then the non-metal
Thank You
H
C
N
O
S
Questions & Discussion
We have explored the fundamental concepts of chemical formulae and
equations, from understanding element symbols to balancing complex
reactions. These skills form the foundation of chemistry.
Key Takeaways
Chemical symbols and formulae are the language of chemistry, allowing us to
represent elements and compounds precisely.
Relative atomic and molecular masses enable us to understand the quantitative
aspects of chemical reactions.
Balanced chemical equations reflect the law of conservation of mass and show us
exactly what happens during chemical reactions.