Bangladesh School Muscat
Year : 10
Subject : Physics
Atoms and Radioactivity
Atomic Structure
The Atom
Specification Point 7.2
Describe the structure of an atom in terms of protons, neutrons and electrons and
use symbols such as 614C to describe particular nuclei.
Atoms are made up of three different particles:
Protons and neutrons are found in the nucleus of an atom, while electrons orbit at a distance.
The properties of each of these particles are shown in the table below:
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Atomic Symbols
The atom shown in the diagram can also be represented using an atomic symbol:
Atomic symbols, like the one above, describe the constituents of nuclei
The top number is called the mass number, and is equal to the total number of particles (protons
and neutrons) in the nucleus.
The lower number is called the atomic number and is equal to the total number of protons in the
nucleus.
Protons, Neutrons and Electrons
Specification Point 7.3
Know the terms atomic (proton) number, mass (nucleon) number and isotope.
When given an atomic symbol, you can figure out the total number of protons, neutrons and
electrons in the atom:
o Protons: The number of protons is equal to the atomic number (also known as the proton
number).
o Electrons: Atoms are neutral, and so in a neutral atom the number of negative electrons
must be equal to the number of positive protons.
o Neutrons: The number of neutrons can be found by subtracting the atomic number from
the mass number (also known as the nucleon number).
Although atoms of the same element always have the same number of protons (and hence
electrons), the number of neutrons can vary:
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The three atoms shown above are all forms of hydrogen, but they each have different numbers of
neutrons
The number of neutrons in an atom does not affect the chemical properties of an atom, only its
mass. Such atoms are called isotopes:
Isotopes are atoms (of the same element) that have equal numbers of protons but different
numbers of neutrons
Radiation
Instability
Specification Point 7.4
Know that alpha (α) particles, beta (β−) particles, and gamma (γ) rays are
ionising radiations emitted from unstable nuclei in a random process.
Some isotopes are unstable – usually because of their large size or because the number of
protons and neutrons within them are out of balance.
As a result, these isotope will decay – emitting little chunks (radiation) in order to reduce
their size or bring them back into balance.
Unstable nuclei decay by emitting high energy particles – radiation
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The resulting radiation can be one of three types: alpha (α) particles, beta (β−) particles, or
gamma (γ) rays, and has very high energy.
If these particles hit other atoms, they can knock out electrons, ionising the atom.
When radiation passes close to atoms, it can knock out electrons, ionising the atom
Ionisation can cause chemical changes in materials, and can damage or kill living cells.
Properties of Radiation
Specification Point 7.5
Describe the nature of alpha (α) particles, beta (β−) particles, and gamma (γ)
rays, and recall that they may be distinguished in terms of penetrating power
and ability to ionise.
Alpha (α) particles are high energy particles made up of 2 protons and 2 neutrons (the
same as a helium nucleus).
They are usually emitted from nuclei that are too large.
Beta (β−) particles are high energy electrons emitted from the nucleus (even though the
nucleus does not normally contain any electrons).
They are usually emitted by nuclei that have too many neutrons.
Gamma (γ) rays are high energy electromagnetic waves.
They are emitted by nuclei that need to lose some energy.
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Alpha particles, Beta particles and Gamma waves
The nature and properties of the different types of radiation are summarised in the table
below.
Note that when you go down the table, the range and penetration increase, but the
ionisation decreases.
Diagram showing the penetrative abilities of alpha (α) particles, beta (β−) particles, and gamma
(γ) rays. Note how some of the gamma rays are able to penetrate the lead
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Practical: The Properties of Radiation
Specification Point 7.6
Practical: Investigate the penetration powers of different types of radiation
using either radioactive sources or simulations.
In your IGCSE examination you may be asked to describe an experiment you could carry out to
investigate the relative penetration of different types of radiation.
In your method you should:
o List any apparatus you need. (Make sure you use the correct names).
o Describe how the apparatus will be set up. (A diagram is helpful).
o Describe how you will carry out a fair test.
o Describe what measurements you will take.
o Describe how you will form your conclusions.
o Describe any safety procedures you will need to follow.
Measuring the relative penetrating ability of different types of radiation
Method:
Connect a Geiger-Muller tube to a counter and, without any sources present, measure
background radiation over a five minute period.
Repeat this three times, and take an average.
Now place a radioactive source a fixed distance of 3 cm away from the tube and take another
reading over a five minute interval.
Now take a set of absorbers: some paper, several different thicknesses of Aluminium (increasing
in 0.5mm intervals) and different thickness of lead.
One at a time, place these absorbers between the source and the tube and take another reading
over a five minute interval.
If the count over that interval falls to background levels (allow for a little random variation),
then the radiation has all been absorbed.
Repeat the above experiment for other radioactive sources.
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Safety:
When not using a source, keep it in a lead lined container.
When in use, try and keep a good distance (a metre or so) between yourself and the source.
When handling the source, do so using tweezers (or tongs) and point the source away from you.
Conclusions:
If the radiation is stopped by paper, the source will be emitting alpha
If the radiation is stopped by a few mm of aluminium (about 5 or 6) then the source is
emitting beta.
If some radiation is still able to penetrate a few mm of lead (5 or 6) then the source is
emitting gamma.
(Note: Some sources emit more than one type of radiation).
Decay
Specification Point 7.7
Describe the effects on the atomic and mass numbers of a nucleus of the
emission of each of the four main types of radiation (alpha, beta, gamma and
neutron radiation).
Alpha Emission
An alpha particle consists of 2 protons and 2 neutrons.
(It is emitted from large unstable nuclei)
An alpha particle consists of two protons and two neutrons
When an alpha particle is emitted from a nucleus:
o The nucleus loses 2 protons:
The atomic (proton) number decreases by 2.
o The nucleus loses 4 particles (nucleons) in total:
The mass (nucleon) number decreases by 4.
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Beta Emission
A beta particle is a high energy electron emitted from the nucleus.
It is emitted when a neutron in the nucleus suddenly changes into a proton – an electron
is created in order to balance the positive charge of the proton.
(Note: The electron is created at the moment of decay – it is not present in the neutron
beforehand).
A beta particle is emitted when a neutron turns into a proton, emitting an electron
When a beta particle is emitted from a nucleus:
o The number of protons in the nucleus increases by 1:
The atomic (proton) number increases by 1.
o The total number of particles in the nucleus remains the same.
The mass (nucleon) number doesn’t change.
Gamma Emission
A gamma waves is emitted from a nucleus when the nucleus needs to lose some energy.
No protons or neutrons are lost from the nucleus in this process:
The atomic (proton) and mass (nucleon) numbers do not change.
Neutron Emission
A small number of isotopes can decay by emitting neutrons.
When a nucleus emits a neutron:
o The number of protons does not change:
The atomic (proton) number does not change.
o The total number of particles (nucleons) in the nucleus decreases by 1:
o The mass (nucleon) number decreases by 1.
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Nuclear Equations
Specification Point 7.8
Understand how to balance nuclear equations in terms of mass and charge.
Nuclear equations, just like chemical equations, balance:
o The sum of the upper (mass) numbers on the left of each equation should equal the
sum on the right.
o The sum of the lower (atomic) numbers should also balance.
Alpha Emission:
Beta Emission:
Neutron Emission:
Note that the beta particle is given an atomic number of -1 in the above examples.
This is because the atomic number is being used to measure charge in this case:
Protons being positive particles, have positive atomic numbers.
Electrons being negative, have a negative number.
