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Lesson 5: Radioactive decay

An atom of any element is made of three tiny particles: protons, neutrons and

electrons. The nucleus of the atom contains protons and neutrons and the

number of protons in the nucleus defines what type of element the atom

is. For example, all atoms of Carbon have 6 protons (the atomic number) and all

atoms of nitrogen have 7 protons. Nearly all the mass of the atom is

concentrated in the nucleus.

Orbiting around the nucleus is a shell of electrons, which are usually equal

in number to the number of protons.

In your exam you may be asked to explain the structure of an atom.

If you look at a periodic table, you may have noticed that the relative atomic

mass (the number in the top left hand corner), is not always a whole number.

The reason for this is that not all atoms of the same element have the same

number of neutrons. Atoms of the same element with different amounts of

neutrons are called isotopes, for example:

1. There are two possible isotopes of oxygen, Oxygen-17 (9 neutrons) and

Oxygen-18 (10 neutrons). These two atoms are isotopes of oxygen and all

must have 8 protons, but each has a different number of neutrons.

What you need to know:

1. The structure of an atom.

2. Understand the process of radioactive decay.

3. What the three types of radioactive decay.

Proton

Neutron Electron

Nucleus

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2. There are two isotopes of carbon, Carbon-11 (5 neutrons) and Carbon-14

(8 neutrons). These two atoms are isotopes of carbon and have 6 protons,

but have different numbers of neutrons.

In your exam you may be asked what an isotope is.

Certain atoms emit ionizing radiation because their nucleuses are unstable and

they can decay (change). This is called radioactive decay and this is a change

inside the nucleus of the atom.

There are three different types of radioactive decay

Type of decay What is emitted What is happening

Alpha (α) A helium nucleus (2

protons and 2

neutrons). This is called

an alpha particle in

radioactive decay.

The nucleus decays, ejecting an

Alpha particle. A new atom is

formed which contains two fewer

protons and neutrons. This new

atom is a different element to the

one you started with.

Beta (β) A high speed electron. The nucleus decays, changing a

neutron into a proton and an

electron. A new atom is formed with

1 less neutron and 1 more proton.

This new atom is a different element

to the one you started with.

Gamma (γ) Electromagnetic

radiation

Sometimes an atom simply contains

too much energy which it needs to

emit, so it gives off a Gamma ray.

The nucleus remains unchanged.

Alpha decay

Unstable nucleus

New nucleus Alpha particle

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Beta decay

Gamma decay

In your exam you may be asked to identify and explain the three different types

of radioactive decay.

When an element occurs in a form where its nucleus is unstable, leading to

radioactive decay, it is called a radioactive isotope.

Radioactive isotopes are used in hospitals to produce scans and to cure diseases,

but radioactive isotopes are hard to find naturally, as they have decayed.

Therefore, hospitals get supplies of radioactive isotopes from nuclear reactors,

where radioactive isotopes are a by-product of the processes that are happening

in a nuclear reactor.

In your exam you might be asked to describe what a radioactive isotope is. You

may also be asked where hospitals get their supplies of radioactive isotopes.

Recap:

1. An atom has a nucleus which contains protons and neutrons and has a shell of

electrons orbiting around the nucleus.

2. Isotopes are atoms of the same element, which have different numbers of neutrons.

3. Radioactive decay occurs when unstable atoms decay, emitting ionizing radiation.

4. There are three different types of radioactive decay, Alpha (α), Beta (β) and Gamma

(γ).

New nucleus Beta particle

Unstable nucleus

Unstable nucleus Gamma wave