SUB-STRAND 1 · MATERIALS

Atoms and Atomic Structure

What makes the different elements around us different?

Look at a piece of iron, a drop of water, a grain of salt or a metal spoon. Everything is made from matter, but what is matter made of?

Start Exploring

Learning Outcomes

By the end of this chapter, you should be able to:

describe an atom as composed of sub-atomic particles;
identify protons, electrons and neutrons and their locations;
state the electrical charges of the sub-atomic particles;
distinguish atomic number from mass number;
determine the numbers of protons, neutrons and electrons in an atom;
explain how elements are arranged in the periodic table according to proton number;
draw simple electron distributions;
explain the formation of ions; and
describe a molecule as a combination of atoms.

01 · Let's Explore

Imagine you have a piece of iron. You cut it into a smaller piece, then an even smaller piece. You can keep dividing it, but eventually you reach particles that are far too small to see with your eyes.

What do you think those tiny particles are like?

What Do You Think?

Tap a question to reveal a scientific clue.

02 · Let's Learn

What is an atom?

An atom is a tiny unit of an element. Atoms contain smaller particles called sub-atomic particles.

The three sub-atomic particles you need to know are:

ProtonPositive (+)In the nucleus

NeutronNo charge (0)In the nucleus

ElectronNegative (−)Outside the nucleus

Inside the atom

The centre of an atom is called the nucleus. The nucleus contains protons and neutrons. Electrons are found outside the nucleus.

Nucleus
p⁺ + n⁰

e⁻e⁻e⁻e⁻

[Diagram showing a nucleus with protons and neutrons, surrounded by electron shells.]

Atomic number and mass number

The atomic number tells us the number of protons in the nucleus of an atom.

The mass number is the total number of protons and neutrons in the nucleus.

Atomic number = protonsMass number = protons + neutrons

Example

Suppose an atom has 11 protons and 12 neutrons.

Atomic number = 11

Mass number = 11 + 12 = 23

Find the particles

For a neutral atom:

Protons = atomic numberElectrons = protonsNeutrons = mass number − atomic number

Atomic number Mass number

Enter values and calculate.

Electron distribution

Electrons are distributed around the nucleus in shells. At this level, use simple shell distributions to represent the arrangement of electrons in atoms.

Hydrogen1

Helium2

Lithium2, 1

Carbon2, 4

Oxygen2, 6

Neon2, 8

Ions

Atoms can become electrically charged when electrons are lost or gained. A charged atom is called an ion.

Neutral atom→loses electrons→positive ion

Neutral atom→gains electrons→negative ion

Think: Why does losing a negatively charged electron make an atom more positive?

Molecules

A molecule is a combination of atoms.

Atoms can combine to form molecules. For example, a water molecule contains hydrogen and oxygen atoms.

H—O—H

[Diagram showing a simple H–O–H representation of a water molecule.]

03 · Explore an Atom

p⁺n⁰p⁺n⁰

Tap a particle to learn about it.

04 · Let's Investigate

Investigation: Build a model atom

Materials: paper, bottle tops or small safe objects, a pencil and a sheet of card.

Draw a circle for the nucleus.
Place labels for protons and neutrons inside the nucleus.
Draw circles around the nucleus to represent electron shells.
Place electron symbols on the shells according to the atom you are modelling.
Label the proton, neutron, electron and nucleus.

Discuss: How does your model show that protons and neutrons are located differently from electrons?

Observation and reasoning

Use the interactive calculator above to choose an atomic number and mass number. Then explain how you determined the numbers of protons, neutrons and electrons.

05 · The Periodic Table

Elements are arranged in the periodic table according to the number of protons in the nuclei of their atoms.

Tap an element to see its proton number.

Think Like a Scientist

Two elements have different atomic numbers. What must be different about the atoms of those elements?

06 · Science Around Us

Cooking and food

The substances in food are made from atoms and combinations of atoms.

Farming

Soil, water, fertilisers and crops contain matter made from elements and compounds.

Technology

Materials used in tools, machines and electronics depend on the properties of elements and their combinations.

Health

Substances used in the human body and healthcare are made from atoms and combinations of atoms.

07 · Did You Know?

Different elements have different numbers of protons.

Electrons have a negative charge.

Protons have a positive charge.

Neutrons have no electrical charge.

The atomic number identifies an element by its number of protons.

08 · New Words

AtomA tiny unit of an element.

ProtonA positively charged sub-atomic particle in the nucleus.

NeutronA sub-atomic particle with no electrical charge in the nucleus.

ElectronA negatively charged sub-atomic particle found outside the nucleus.

NucleusThe central part of an atom containing protons and neutrons.

Atomic numberThe number of protons in the nucleus.

Mass numberThe total number of protons and neutrons.

IonAn electrically charged atom formed when electrons are gained or lost.

MoleculeA combination of atoms.

09 · Check Your Understanding

10 · Practical Challenge

Choose one element from the interactive mini periodic table. Find its atomic number and use it to determine the number of protons in a neutral atom. If you are given a mass number, determine the number of neutrons. Then draw a simple electron distribution.

I selected an element. I identified its atomic number. I determined the number of protons. I determined the number of electrons for a neutral atom. I drew its electron distribution.

11 · Project Work

Build and Explain an Atom Model

Work individually or in groups. Select an element, research or use class information to identify its proton number, and build a simple physical or paper model showing its nucleus and electron distribution.

Present your model to the class and explain:

the element selected;
the number of protons;
the number of electrons in a neutral atom;
the number of neutrons if a mass number is provided; and
how the model represents the atom.

12 · Key Points

Atoms are tiny units of elements.
Atoms contain protons, neutrons and electrons.
Protons are positive, neutrons have no charge and electrons are negative.
Protons and neutrons are in the nucleus; electrons are outside it.
Atomic number equals the number of protons.
Mass number equals protons plus neutrons.
For a neutral atom, the number of electrons equals the number of protons.
Elements are arranged in the periodic table according to proton number.
Atoms can gain or lose electrons to form ions.
A molecule is a combination of atoms.

13 · End-of-Chapter Exercises

Section A · Multiple Choice

Which particle has a positive charge? Proton.
Which particle has no charge? Neutron.
Which particle is found outside the nucleus? Electron.
What does atomic number represent? The number of protons.
What is mass number? Protons + neutrons.
What happens when an atom gains or loses electrons? It can become an ion.

Section B · Short Answer

Describe the structure of an atom.
State the charges of the three sub-atomic particles.
Distinguish between atomic number and mass number.
Describe a molecule.

Section C · Application

An atom has atomic number 8 and mass number 16. Determine its protons, neutrons and electrons if it is neutral.
An atom has 12 protons. What is its atomic number?
Explain why two elements with different atomic numbers have different numbers of protons.

Section D · Investigation

Build a model of an atom using safe locally available materials. Label its nucleus, protons, neutrons and electron shells. Explain what each part represents.

Section E · Challenge Question

Two neutral atoms have the same number of electrons but are said to be different elements. Is this possible under the basic model used in this chapter? Explain your reasoning using atomic number.

14 · Chapter Summary

Atoms are tiny units of elements. They are composed of sub-atomic particles: protons, neutrons and electrons. Protons and neutrons are found in the nucleus, while electrons are found outside the nucleus.

The atomic number is the number of protons in an atom. The mass number is the total number of protons and neutrons. For a neutral atom, the number of electrons equals the number of protons.

Elements are arranged in the periodic table according to the number of protons in the nuclei of their atoms. Electrons can be gained or lost, causing atoms to form ions. Atoms can also combine to form molecules.

REMEMBER

The number of protons in an atom identifies its element, while the arrangement of its sub-atomic particles helps us understand its structure and behaviour.