Introduction

Look around you. Everything you can see, touch, or hold is made of matter. The air you breathe, the water you drink, the desk you sit at, and even your own body are all forms of matter. Although these substances appear very different, scientists understand them using a single idea called the particle model of matter.

The particle model explains that all matter is made of tiny particles that are constantly moving. The way these particles are arranged and how they move determines whether a substance is a solid, liquid, or gas. This simple model helps explain many everyday observations, from ice melting in a drink to the smell of perfume spreading across a room.

What Is Matter?

Matter is anything that:

• has mass
• occupies space (has volume)

Almost everything around us is matter.

Examples include:

• air
• water
• rocks
• metals
• plants
• animals
• plastic

Things that are not matter include:

• light
• sound
• heat
• electricity

These are forms of energy, not matter.

Definition:
Matter is anything that has mass and occupies space.

 

 

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The Particle Model of Matter

Scientists explain the behaviour of matter using the particle model.

The particle model states that:

• all matter is made of tiny particles,
• particles are constantly moving,
• there are spaces between particles,
• particles attract one another,
• heating increases particle movement,
• cooling decreases particle movement.

Although we cannot usually see individual particles, their behaviour explains many properties of matter.

Solids

In a solid, particles are packed very closely together in a regular arrangement.

Properties of Solids

• fixed shape
• fixed volume
• particles vibrate about fixed positions
• strong attractive forces between particles
• difficult to compress

Examples include:

• ice
• wood
• iron
• glass

Because the particles cannot move past one another, solids keep their shape.

Liquids

In a liquid, particles remain close together but are free to move around each other.

Properties of Liquids

• fixed volume
• no fixed shape
• flow easily
• slightly compressible
• particles move continuously

Examples include:

• water
• milk
• oil
• mercury

Liquids take the shape of their container because their particles can slide past one another.

Gases

In a gas, particles are far apart and move rapidly in all directions.

Properties of Gases

• no fixed shape
• no fixed volume
• highly compressible
• particles move freely
• fill any container

Examples include:

• oxygen
• nitrogen
• carbon dioxide
• helium

Because gas particles are widely spaced, gases can expand easily.

 

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Comparing the Three States

Changes of State

Matter can change from one state to another when energy is added or removed.

During these changes:

• particles themselves do not change,
• only their energy, spacing, and movement change.

Particle Behaviour During Changes of State

Melting

When a solid is heated:

• particles gain energy,
• vibrations become stronger,
• attractive forces weaken,
• particles begin moving past one another.

The solid becomes a liquid.

Boiling

Further heating causes:

• particles to gain even more energy,
• particles to overcome attractive forces,
• particles to spread far apart.

The liquid becomes a gas.

Freezing and Condensation

Cooling removes energy.

Particles:

• move more slowly,
• come closer together,
• become more strongly attracted.

Liquids freeze into solids, and gases condense into liquids.

 

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Diffusion

Diffusion is the movement of particles from an area of high concentration to an area of low concentration.

This occurs because particles are constantly moving.

Examples include:

• perfume spreading through a room,
• food colouring spreading through water,
• oxygen moving into the bloodstream,
• carbon dioxide leaving the lungs.

Diffusion occurs fastest in gases because gas particles move most rapidly.

Everyday Example of Diffusion

If someone sprays perfume at one side of a room, people across the room can smell it a short time later.

Why?

The perfume particles move randomly through the air until they become evenly spread throughout the room.

 

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Expansion

Heating causes particles to move faster.

Although the particles themselves do not become larger, the spaces between them increase, causing most substances to expand.

Examples include:

• railway tracks with expansion gaps,
• bridges with expansion joints,
• hot air balloons rising,
• metal lids loosening when warmed.

Cooling has the opposite effect, causing particles to move more slowly and come closer together.

Worked Example 1

A metal rod becomes slightly longer after being heated.

Question

Does each metal particle become larger?

Solution

No.

The particles remain the same size.

Heating increases their kinetic energy, causing them to vibrate more and move slightly farther apart.

This increases the overall length of the rod.

Worked Example 2

A student notices that food colouring spreads through a glass of still water without stirring.

Question

Use the particle model to explain this observation.

Solution

Water particles and food-colouring particles are constantly moving.

The food-colouring particles move randomly from areas of high concentration to areas of low concentration until they are evenly distributed throughout the water.

This process is called diffusion.

Real-World Connection

The particle model helps scientists and engineers design many everyday technologies. Refrigerators and air conditioners rely on changes of state to transfer heat, while aerosol sprays work because gases expand to fill available space. Engineers include expansion joints in bridges and railway tracks to allow materials to expand safely on hot days. Understanding particle behaviour is also essential in chemistry laboratories, medicine, food preservation, and manufacturing.

Did You Know?

Scientists estimate that a single breath of air contains trillions upon trillions of particles. Because gas particles move continuously, some of the air you breathe today may contain particles once breathed by dinosaurs, ancient Egyptians, or famous historical figures. Over millions of years, atmospheric gases become thoroughly mixed through diffusion and global air circulation!

Key Terms

• Matter — anything that has mass and occupies space.
• Particle model — a model describing matter as tiny particles in constant motion.
• Solid — a state of matter with fixed shape and fixed volume.
• Liquid — a state of matter with fixed volume but no fixed shape.
• Gas — a state of matter with neither fixed shape nor fixed volume.
• Diffusion — the movement of particles from high concentration to low concentration.
• Expansion — an increase in volume caused by particles moving farther apart.
• Change of state — the process by which matter changes between solid, liquid, and gas.

Key Takeaways

• Matter is anything that has mass and occupies space.
• The particle model explains that all matter is made of tiny particles that are constantly moving.
• Solids, liquids, and gases differ because of the arrangement and movement of their particles.
• During changes of state, particles gain or lose energy, changing their movement and spacing without changing their identity.
• Diffusion and thermal expansion can both be explained using the particle model.
• The particle model provides the foundation for understanding many topics in chemistry, physics, biology, and everyday life.

Introduction

Everything around us—from the air we breathe to the stars in the night sky—is made of incredibly tiny particles called atoms. Although atoms are far too small to be seen with the naked eye, they are the basic building blocks of all matter.

Our understanding of atoms has changed dramatically over the past 2,000 years. Early scientists believed atoms were tiny solid spheres, but modern experiments have revealed that atoms have a complex internal structure consisting of a dense nucleus surrounded by rapidly moving electrons. Understanding atomic structure is one of the foundations of chemistry because it explains why different elements have different properties and how atoms form chemical bonds.

What Is an Atom?

An atom is the smallest particle of an element that still retains the properties of that element.

Atoms combine to form:

• molecules
• compounds
• all living organisms
• all everyday materials

Everything made of matter contains atoms.

Definition:
An atom is the smallest unit of an element that retains the chemical properties of that element.

 

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The Historical Development of the Atomic Model

Scientists developed the modern atomic model over many centuries as new evidence became available.

1. Democritus (About 400 BCE)

The ancient Greek philosopher Democritus suggested that matter could be divided only until reaching tiny, indivisible particles called atomos, meaning "uncuttable."

However:

• he had no experimental evidence,
• his ideas were largely ignored for many centuries.

2. Dalton's Atomic Theory (1803)

In the early 1800s, John Dalton proposed the first scientific atomic theory.

Dalton suggested that:

• all matter is made of atoms,
• atoms of the same element are identical,
• atoms cannot be created or destroyed during chemical reactions,
• compounds form when atoms combine in fixed ratios.

Dalton pictured atoms as solid spheres, similar to tiny billiard balls.

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3. Thomson's Plum Pudding Model (1897)

After discovering the electron, J. J. Thomson realised that atoms were not indivisible.

He proposed the plum pudding model, in which:

• the atom was a positively charged sphere,
• negatively charged electrons were embedded throughout it,
• there was no nucleus.

Although this model was later shown to be incorrect, it was an important step in understanding atomic structure.

4. Rutherford's Nuclear Model (1911)

Ernest Rutherford performed the famous gold foil experiment.

His results showed that:

• most of the atom is empty space,
• nearly all the mass is concentrated in a tiny nucleus,
• electrons occupy the space surrounding the nucleus.

This completely changed scientists' understanding of atoms.

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5. Bohr's Planetary Model (1913)

Niels Bohr suggested that electrons move around the nucleus in specific energy levels (shells).

His model successfully explained many properties of hydrogen.

Although modern atomic theory is more sophisticated, Bohr's model is still widely used as a simple introduction to atomic structure.

6. The Modern Atomic Model

Today, scientists understand that:

• the nucleus contains protons and neutrons,
• electrons do not travel in fixed circular paths,
• electrons occupy regions of space called the electron cloud, where there is a high probability of finding them.

This modern model is supported by many experiments and forms the basis of modern chemistry and physics.

The Structure of an Atom

Every atom has two main regions:

• the nucleus
• the electron cloud

The Nucleus

The nucleus is:

• extremely small,
• very dense,
• positively charged overall,
• contains nearly all the atom's mass.

It contains:

• protons
• neutrons

The Electron Cloud

Surrounding the nucleus is the electron cloud.

This region contains:

• electrons,
• almost all of the atom's volume,
• very little of its mass.

Electrons move extremely rapidly and occupy regions where they are most likely to be found rather than following fixed circular paths.

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The Three Main Subatomic Particles

Atoms are made of three main subatomic particles.

Protons

• positively charged
• found in the nucleus
• determine the identity of an element

Neutrons

• no electrical charge
• found in the nucleus
• add mass and help stabilise the nucleus

Electrons

• negatively charged
• found in the electron cloud
• responsible for chemical bonding and electricity

Comparing the Atomic Models

Each new model improved upon the previous one by explaining observations that earlier models could not.

Worked Example 1

A student says:

"Electrons are found inside the nucleus."

Question

Is this correct?

Solution

No.

Electrons are found in the electron cloud surrounding the nucleus.

The nucleus contains only protons and neutrons.

Worked Example 2

Which subatomic particle has:

• a positive charge?
• no charge?
• a negative charge?

Answer

• Proton → positive
• Neutron → neutral
• Electron → negative

Why Understanding Atomic Structure Is Important

Atomic structure helps explain:

• why different elements have different properties,
• how atoms form chemical bonds,
• why materials conduct electricity,
• how chemical reactions occur,
• how radioactive decay works.

Nearly every topic in chemistry builds on the ideas introduced in this lesson.

Real-World Connection

Modern technologies rely on our understanding of atomic structure. Medical imaging techniques such as PET scans and radiation therapy use knowledge of atomic nuclei, while semiconductor devices in computers and smartphones depend on the behaviour of electrons. Even solar panels generate electricity by controlling the movement of electrons within atoms.

Did You Know?

If an atom were enlarged until its nucleus was about the size of a marble, the outer edge of the electron cloud would be hundreds of metres away. This means that most of an atom is empty space, even though solid objects feel completely solid because of the forces between their atoms.

Key Terms

• Atom — the smallest unit of an element that retains its chemical properties.
• Atomic model — a scientific representation of the structure of an atom.
• Nucleus — the tiny, dense centre of an atom containing protons and neutrons.
• Electron cloud — the region around the nucleus where electrons are most likely to be found.
• Proton — a positively charged particle found in the nucleus.
• Neutron — a neutral particle found in the nucleus.
• Electron — a negatively charged particle found in the electron cloud.
• Subatomic particle — a particle smaller than an atom.

Key Takeaways

• Atoms are the fundamental building blocks of all matter.
• Scientific understanding of atoms has developed over centuries through experimentation.
• The modern atom consists of a dense nucleus surrounded by an electron cloud.
• Protons and neutrons are located in the nucleus, while electrons occupy the surrounding electron cloud.
• Modern atomic theory is based on experimental evidence and explains the structure and behaviour of matter far better than earlier models.
• Understanding atomic structure provides the foundation for studying elements, chemical bonding, reactions, and many applications in science and technology.