Chapter 3: Atoms and Molecules
3.1 Laws of Chemical Combination
Law of Conservation of Mass
Mass can neither be created nor destroyed in a chemical reaction. The total mass of reactants equals the total mass of products.
Law of Constant Proportions
In a chemical substance, the elements are always present in definite proportions by mass.
3.2 What is an Atom?
Atoms are the building blocks of all matter. They are extremely small, measured in nanometres (1 nm = 10⁻⁹ m).
Aluminium: Al
Carbon: C
Hydrogen: H
Iron: Fe
Sodium: Na
Oxygen: O
3.3 Molecules
A molecule is the smallest particle of an element or a compound that is capable of an independent existence.
- Atomicity: The number of atoms constituting a molecule.
- Ion: A charged species (Anion is negative, Cation is positive).
3.4 Writing Chemical Formulae
We use the Criss-Cross Method to write formulas based on valency.
Example: Magnesium Chloride
MgCl₂
(Mg²⁺ and Cl⁻)
3.5 Molecular Mass
Sum of the atomic masses of all the atoms in a molecule.
Example: Water (H₂O)
(2 × 1 u) + (16 u) = 18 u
(2 × 1 u) + (16 u) = 18 u