1. Lesson 5: Radioactive decay
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.
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.
Electron
Neutron
Proton Nucleus
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.
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2. 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 The nucleus decays, ejecting an
protons and 2 Alpha particle. A new atom is
neutrons). This is called formed which contains two fewer
an alpha particle in protons and neutrons. This new
radioactive decay. 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 Sometimes an atom simply contains
radiation too much energy which it needs to
emit, so it gives off a Gamma ray.
The nucleus remains unchanged.
Alpha decay
New nucleus Alpha particle
Unstable nucleus
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3. Beta decay
New nucleus Beta particle
Unstable nucleus
Gamma decay
Unstable nucleus Gamma wave
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).
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