Chapter 8: Atoms and Molecules

1. Matter

Matter is anything that has:

  • Mass

  • Occupies space

All matter is made up of tiny particles called atoms and molecules.

2. Atoms

What is an Atom?

An atom is the smallest particle of an element that has the chemical properties of that element.

Structure of an Atom

Particle

Charge

Location

Proton

Positive (+)

Nucleus

Neutron

No charge (0)

Nucleus

Electron

Negative (-)

Electron shells

Important Facts

  • Nucleus contains protons and neutrons.

  • Electrons move around the nucleus in shells.

  • In a neutral atom:

    Number of protons = Number of electrons

3. Elements

What is an Element?

An element is a pure substance made up of only one type of atom.

Examples

Element

Symbol

Hydrogen

H

Oxygen

O

Carbon

C

Iron

Fe

Copper

Cu

Sodium

Na

Key Point

All atoms of the same element have the same number of protons.

4. The Periodic Table

The periodic table is a table that shows all known elements.

Information Found

  • Element name

  • Chemical symbol

  • Proton number (atomic number)

Example

Oxygen (O)

  • Proton number = 8

  • 8 protons

  • 8 electrons

5. Molecules

What is a Molecule?

A molecule is formed when two or more atoms join together chemically.

Examples

Molecule

Formula

Hydrogen

H₂

Oxygen

O₂

Water

H₂O

Carbon dioxide

CO₂

Key Point

Molecules can contain:

  • Same type of atoms (O₂)

  • Different types of atoms (H₂O)

6. Compounds

What is a Compound?

A compound is a substance made up of two or more different elements chemically combined in a fixed ratio.

Examples

Compound

Elements Present

Water (H₂O)

Hydrogen + Oxygen

Carbon dioxide (CO₂)

Carbon + Oxygen

Sodium chloride (NaCl)

Sodium + Chlorine

Properties of Compounds

  • Different from the elements that form them.

  • Cannot be separated by physical methods.

Example:

  • Hydrogen burns.

  • Oxygen supports burning.

  • Water extinguishes fire.

7. Mixtures

What is a Mixture?

A mixture contains two or more substances physically mixed together.

Examples

  • Air

  • Salt water

  • Soil

Properties of Mixtures

  • No fixed composition.

  • Components keep their own properties.

  • Can be separated by physical methods.

8. Difference Between Elements, Compounds and Mixtures

Element

Compound

Mixture

One type of atom

Different elements chemically combined

Substances physically mixed

Pure substance

Pure substance

Not pure

Cannot be broken down chemically

Can be broken down chemically

Can be separated physically

9. Chemical Formulae

A chemical formula shows:

  • Types of atoms present

  • Number of each atom

Examples

Formula

Meaning

H₂O

2 H atoms, 1 O atom

CO₂

1 C atom, 2 O atoms

NH₃

1 N atom, 3 H atoms

Rule

The small number written below is called a subscript.

Example:

  • O₂ = 2 oxygen atoms

  • H₂ = 2 hydrogen atoms

10. Chemical Equations

Chemical equations show chemical reactions.

Example

Hydrogen + Oxygen → Water

2H₂ + O₂ → 2H₂O

Important Rule

Number of atoms must be the same on both sides.

This is called the Law of Conservation of Mass.

11. Law of Conservation of Mass

During a chemical reaction:

  • Mass is neither created nor destroyed.

  • Total mass of reactants = Total mass of products.

Example

10 g Hydrogen + 80 g Oxygen → 90 g Water

Total mass before reaction = Total mass after reaction

12. Atomic Models

Scientists developed different models to explain atoms.

Dalton's Model

  • Atom is a solid sphere.

Thomson's Model

  • Positive sphere with electrons embedded.

Rutherford's Model

  • Small positive nucleus.

  • Mostly empty space.

Bohr's Model

  • Electrons move in shells around nucleus.

Common Exam Questions

Define an atom.

The smallest particle of an element that retains its chemical properties.

Define a molecule.

Two or more atoms chemically combined together.

Define an element.

A pure substance made up of only one type of atom.

Define a compound.

A substance formed when two or more different elements chemically combine in a fixed ratio.

Define a mixture.

Two or more substances physically mixed together.

Super Summary (Must Memorise)

  1. Matter is made of atoms and molecules.

  2. Atom = smallest particle of an element.

  3. Proton (+), neutron (0), electron (-).

  4. Element = one type of atom only.

  5. Molecule = two or more atoms joined together.

  6. Compound = different elements chemically combined.

  7. Mixture = substances physically mixed.

  8. Formula shows types and numbers of atoms.

  9. Mass is conserved in chemical reactions.

  10. Total mass of reactants = total mass of products.