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FIND OUT MORE. Electricity 182 • Elements 160–161 • Nuclear Energy 167

Science and Technology^157

All matter is made from particles called atoms. The

Ancient Greeks described atoms as the smallest particles

that make up everything. We now know that atoms are

built from even smaller particles. Atoms link together

and make. MOLECULES.

Most of an atom is empty space. Protons and neutrons

occupy the nucleus at the centre of the atom. Electrons

orbit the nucleus like planets around a star. They are

grouped in layers called shells.

HOW CAN ATOMS BE SPLIT?

Protons and neutrons are held together in the nucleus at

the centre of the atom by a strong force. But this force

can be overcome by striking the nucleus with a neutron,

a proton, or another particle. The nucleus may split and

form new atoms. Atoms are split in this way inside

nuclear reactors and during nuclear explosions.

HOW BIG IS AN ATOM?

The radius of a typical atom is one tenth of a billionth

of a metre. A string of atoms one metre long contains

one atom for every person in the world. A cube of

sugar contains as many atoms as there are stars in the

Universe. The biggest atom (caesium) is about nine

times the diameter of the smallest atom (helium).

WHAT’S INSIDE AN ATOM?

The particles that make up atoms are electrons,

protons, and neutrons. Their position in an atom is the

. ATOMIC STRUCTURE. Electrons are a type of
lepton. Protons and neutrons are made up of three
quarks each. Quarks and leptons are fundamental
particles – the smallest particles in the Universe.

WHAT SHAPES ARE MOLECULES?

The simplest molecules consist of just two atoms, and are shaped like dumb-bells.

But atoms can link together to make molecules of almost any shape imaginable —

pyramids, chains, rings, spirals, balls, or tubes.

WHAT MAKES ATOMS STICK TOGETHER?

Electrons carry a negative electric charge, protons

carry a positive charge. The attraction between them

holds electrons in orbits. When atoms come together

they share electrons in their outer shells to form

chemical bonds.

Different atoms bonded (stuck) together in particular

arrangements are called molecules. Water molecules, for example,

have two hydrogen atoms bonded to one oxygen atom.

MOLECULES

1 SIMPLE MOLECULE

A single water molecule consists

of three atoms.

4 COMPLEX MOLECULE

Every molecule of the sugar

sucrose is built from 45 atoms

linked in an identical pattern.

ATOMIC BOMB 3

In a nuclear explosion the

atom’s nucleus (centre) is

split. Neutrons are released

and hit other nuclei, setting

off a chain reaction. The result is

a big release of energy in a blast

of heat, light, and radiation.

ATOMIC STRUCTURE

4 CARBON ATOM

This model of a carbon atom is

split in half to show the inside.

The nucleus of a carbon atom

contains six neutrons and six

protons. Six electrons orbit the

nucleus in two shells.

1 SEEING ATOMS
Electron microscopes can magnify
objects 10 million times to show
individual atoms. This picture
shows a clump of gold atoms (red
and yellow) on a regular pattern
of carbon atoms (green).

Atoms

Neutrons (yellow) Protons (green)

SUCROSE

(C 12 H 22 O 11 )

Inner shell

Hydrogen

157157157

WATER (H 2 0)

Oxygen

Carbon

Nucleus

Outer shell

Hydrogen

Oxygen

Electrons

atoms