ATOMIC MODELS

By the end of this chapter, learners will be able to;

a) Understand the structure of an atom in terms of a positive nucleus and negative electrons.
b) Understand the terms: atomic number, mass number, and isotopes, and use them to
represent different nuclides.
c) Understand the methods by which electrons are ejected from matter/atoms and how these
electrons are useful.

INTRODUCTION TO ATOMIC MODELS

This deals with nuclear model of an atom. Just as bricks are the building blocks of a wall, an atom is the
building block of any piece of matter. When atoms join together, they form molecules. Similarly, the
molecules join to form big structures/objects around you.

The structure of an atom: Atomic structure refers to the arrangement of sub-atomic elements in the
atom. An atom is the smallest particle of an element that can take part in a chemical reaction. An atom
consists of three subatomic particles and these are;

i) Neutrons.
ii) Protons.
iii) Electrons.

An atom is made of a central part called the nucleus around which electrons rotate. The nucleus is
positively charged because it consists of protons which are positively charged and neutrons which have
no charge. The properties of the subatomic particles of an atom are as shown in the table below.

Name Symbol Mass Charge

Protons P 1 Positive
Neutrons n 1 No charge
Electrons e 0 Negative

Note: The number of protons in the nucleus is equal to the number electrons around the nucleus and
since they have opposite charges the atom has no charge.

Atomic number, z: This is the number of protons in the nucleus of an atom.

Atomic number, z = Number of protons

Mass number [atomic mass], A: This is the total number of protons and neutrons in the nucleus of an
atom.

Mass number = Number of protons + Number of neutrons

A=z+n
A
If an atom of an element X is represented as Z X

Then z = Atomic number and A = Atomic mass

 35
Example: Given the chloride atom 17 Cl. Find the number of neutrons and electrons in the atom

A=z+n

Where A = 35 and z = 17

n = 35 – 17

= 18 neutrons

Number of electrons = Number of protons

= Atomic number, z

= 17 electrons

TASK: visit your learner’s guide and attempt the followings;

a) Example 5.1 on page 70

b) Exercise 5.2 on page 70

Charged atoms

Recall that an electron carries a negative charge while protons carries positive charge. These two
charges are equal but opposite. When an atom has the same number of electrons and protons, it has
equal number of negative charges and positive charges. Therefore, the charges cancel and the atom is
said to be neutral atom and at times, an atom may gain or lose electrons.

Note:

a) If an atom loses an electron, it now has more protons than electrons and therefore, it acquire a
positive charge
b) If an atom gains an electron, it will have more electrons than protons and therefore, it acquire a
negative charge
c) Always remember that a charged atom is called an ion

An ion can be represented as shown below:

A charge of negative two means that the element has gained two electrons

Task: visit your learner’s guide and attempt the followings;

a) Example 5.2 on page 71

b) Exercise 5.3 on page 71
ISOTOPES:

These are atoms of the same element with the same atomic number but different mass numbers.
Therefore isotopes of an element have the same number of protons and electrons and different number
of neutron.

Examples of isotopes are;

35 36 37
Chlorine; 17 Cl 17 Cl 17 Cl
12 13 14
Carbon; 6 C 6 C 6 C
16 17 18
Oxygen; 8 O 8 O 8 O
1 2 3
Hydrogen; 1 H 1 H 1 H

Table below representing isotopic forms of hydrogen

Isotopes of hydrogen No. of protons No. of neutrons Symbol

1
Hydrogen-1 1 0 1 H
2
Hydrogen-2 1 1 1 H(D)
3
Hydrogen-3 1 2 1 H(T)

Complete tables below:

(a) Isotopic forms of carbon

Name of Symbol No. of protons No. of Atomic Atomic mass

isotope neutrons number
12
Carbon-12 6 C
Carbon-13 6
Carbon-14 8
(b) Isotopic forms of oxygen

Name of Symbol No. of protons No. of Atomic Atomic mass

isotope neutrons number
Oxygen-16 8
Oxygen-17 17
16
Oxygen-18 8 O
Uses of isotopes

Isotopes of different elements have different applications. For examples;

1. An isotope of uranium(92235U) is used in nuclear power plants to get electricity

2. For medical purposes, an isotope of:
i) Cobalt is used in the treatment of cancer
ii) Iodine is used in treatment of goiter

A nuclide
A nuclide is the nucleus of an atom of a specific isotope. A nuclide is characterized by the number of
positively charged protons (Z), neutrons (N) and the energy state of the nucleus. Examples of nuclides
include:

(i) Chlorine-37, an isotope of chlorine: the nucleus consist of 17 protons and 20 neutrons
(ii) Sodium-23, an isotope of sodium: the nucleus consists of 11 protons and 12 neutrons
(iii) Chlorine-35, an isotope of chlorine: the nucleus consists of 17 protons and 18 neutrons

METHODS BY WHICH ELECTRONS ARE EJECTED FROM ATOMS

Electrons are normally ejected or released from metal surfaces (emissive surfaces) whenever they gain
sufficiently enough energy to overcome the attractive inward force of the positively charged nucleus.
They do it through the following methods;

(a) Thermionic emission

(b) Photoelectric emission

THERMIONIC EMISSION

This is the process by which electrons are emitted from the metal surface by application of heat energy.
Applications of thermionic emission: Thermionic emission can be applied in the following devices;

 Diode valves.

 Cathode ray tube.

 Cathode ray oscilloscope.

 X-ray tube.

The diode valve: This is an electrical device that conducts electricity in one direction only. There are two
types of diodes and these are

i) Semi-conductor diode.
ii) Vacuum diode.

Thermionic diode valve: This is an electrical device that uses the principle of thermionic emission. This
is sometimes referred to as vacuum diode. It consists of the following;

i) Evacuated glass envelope to prevent electrons from colliding with air molecules.
ii) The anode which accelerates electrons emitted from the heated cathode.
iii) The cathode which emits electrons thermionically.
iv) The heater which heats the cathode electrically.
Illustration
Action of a diode:

 When the cathode, C is heated, it emits electrons by thermionic emission.

 When no voltage is applied across the anode and the cathode, the emitted electrons stay around the
cathode.

 If the anode, A is positive with respect to the cathode, C, the electrons from the space charge are
attracted to the anode and this causes the flow of current.

 The number of electrons emitted from the cathode depends on the voltage, that is the higher the
voltage the higher the temperature and more electrons are emitted hence the higher the current in the
circuit. When the voltage increases the current increases until its maximum value, Ia where any
increase in voltage will not increase the current. This maximum value is called saturation current.

RECTIFICATION:

This is a process of changing an alternating current to direct current by use of a diode. There are two
types of rectification and these are;

i) Half wave rectification

ii) Full wave rectification

Half wave rectification:

MODE OF ACTION

 During the first half of the cycle when A is positive with respect to B, the anode is positive with
respect to the cathode and diode conducts.

 Electrons are attracted to the anode and current flows through R.

 During the next half of the cycle when B is positive with respect to A, the anode is negative with
respect to the cathode and the diode does not conduct.

 No electrons are attracted to the anode and therefore no current flows through R.

 Hence current flows through R during only one half of the cycle when A is positive with respect to B.
The graph of voltage against time for half wave is as shown below.

Full wave rectification:

a) Using two diodes:

MODE OF ACTION
 During the first half of the cycle when A is positive with respect to B, diode D2 conducts and
diode D1 does not conduct but current flows through R.
 During the next half of the cycle when B is positive with respect to A diode D1 conducts and
diode D2 does not conduct but current flows through R.
 Hence current flows through R during both cycles and therefore both cycles are rectified giving a
full wave rectification.
b) Using four diodes:
MODE OF ACTION
 During the positive half of the cycle when A is positive with respect to B diodes D1 &D3 conducts
and diodes D2 &D4 do not conduct but current flows through R.
 During the negative half of the cycle when B is positive with respect to A diodes D2 &D4 conduct
and diodes D1&D3 do not conduct but current flows through R.

 Hence current flows through R during both cycles and therefore both cycles are rectified giving a
full wave rectification.
The graph of voltage against time for full wave is as shown below

PHOTO ELECTRIC EMISSION:

This is the process by which electrons are emitted from the metal surface when exposed to
electromagnetic waves of high frequency and shortwave length.
Photo electric emission occurs in phototubes [photoelectric cells]. The electrons emitted are
referred to as photoelectrons and the electromagnetic waves used are ultra violet radiations.
Photoelectric cell:
Photoelectric cell is composed of the cathode and the anode enclosed in a vacuum tube. The glass
tube is evacuated in order to avoid collision of cathode rays with air molecules which may lead to
low current flowing due to loss in kinetic energy of cathode rays.

Mechanism of a photoelectric cell:

Electromagnetic radiation is directed on to the cathode and supplies sufficient energy that causes
the liberation of electrons. The electrons emitted are then attracted to the anode and the flow of
electrons generates a current around the circuit and the ammeter deflects. The amount of the
current is proportional to the intensity of the radiation. The stream of electrons flowing from the
cathode to the anode is referred to as cathode rays.
CATHODE RAYS:
These are streams of electrons moving at a very high speed. They are produced from the cathode
by thermionic emission. Cathode rays carry energy since they possesses speed
Production of cathode rays:

Mode of operation and production

 Cathode rays are produced when the metal cathode is electrically heated using low voltage.
 The cathode rays are then accelerated by the anode which is at a positive potential with respect
to the cathode.
 Some of the electrons pass through the anode and a parallel beam of electrons is obtained which
is received as spot on the fluorescent screen.
 The tube is evacuated to prevent cathode rays from colliding with air particles hence free
movement of cathode rays.
Properties of cathode rays:
 They travel in straight lines.
 They are negatively charged since they are streams of electrons.
 They produce X-rays when stopped by a heavy metal.
 They are deflected by both magnetic and electric fields.
 They possess momentum and energy.
 They cause other materials to give off light [fluorescence].
Applications of cathode rays:
Cathode rays are applied in the following devices;
 Cathode ray oscilloscope.
 X – Ray tube.
 Diode.
Cathode Ray Oscilloscope [C.R.O]:
It is an instrument used to study current and voltage wave forms.
It has three main parts and these are
i) Electron gun.
ii) Deflecting system.
iii) Fluorescent screen.
Functions of the parts:

1. Electron gun: It consists of a heater, cathode, grid and Anodes

i) The heater: This heats the cathode electrically

ii) The cathode: It emits electrons when heated electrically by the heater

iii) The grid: It controls the number of electrons reaching the anode and therefore controls the
brightness of the spot on the screen. The grid is at a negative potential with respect to the cathode
iv) The anodes: These are used to accelerate the electrons produced by the cathode. The anodes
are at a positive potential with respect to the cathode.

2. Deflecting system:

It consists of two pairs of metal plates and these are X – plates and Y – plates. The Y – plates are
horizontal and deflect the beam of electrons vertically while the X – plates are vertical and deflect
the beam of electrons horizontally.

3. Fluorescent screen: This is where the spot of electrons is received.

4. Time base: This is a special circuit that generates p.d which rises steadily to a certain value and
falls rapidly to zero. This provides a saw-toothed voltage to X-plates. Hence the time base is used to
generate a saw-toothed voltage. Vertical motion of the beam causes the beam to travel across the
screen and horizontal motion of the beam provides the wave form of the beam on the screen.

Note:

The time base is connected to the X – plates and causes the spot to move from left to right called
linear sweep and the spot returns to the left before it starts the next sweep called fly back.

Wave forms on C.R.O

i) A.C on the X-plates and time base off

 ii) A.C on the Y-plates and time base off

iii) A.C on the Y-plates and time base on

iv) D.C on the Y-plates only and time base off

v) D.C on the X-plates only and time base of

vi) D.C on the Y-plates and time base on

vii) No potential difference is applied to the Y-plates and time base off

Uses of C.R.O:

 It is used to measure potential difference.

 It is used to study wave forms.

 It is used to measure the frequency of the wave.

 It is used to measure the wave length of the wave.

 It is used to measure phase difference between two voltages.

 It is used as a timing device.

 It is used to measure the peak value of alternating and direct current.

Advantages of C.R.O over ordinary ammeter or voltmeter:

 It has infinite resistance therefore draws no current from the circuit.

 It is not affected by high voltages/currents.

 It measures both alternating and direct voltages.

 It is very accurate.

 It has no coil to burn out.

 It has instantaneous response.

Disadvantages of C.R.O over ordinary ammeter or voltmeter:

 It is bulky.

 It requires skilled personnel.

 It is expensive.

 It takes a lot of time to measure voltages.

 It does not give direct readings.

Advantages of voltmeter or ammeter over C.R.O

 It is portable.

 It does not require skilled personnel.

 It is cheap.

 It takes a short time to measure voltages/currents.

 It gives direct readings.

Disadvantages of voltmeter or ammeter over ordinary C.R.O:

 It draws current from the circuit.

 It is affected by high voltages/currents.

 It measures only direct voltages/currents.

 It is not very accurate.

 It has coil that burns out

X – RAYS:

These are short wave length electromagnetic waves which are produced when cathode rays are
stopped by a heavy metal.

Production of X – Rays [X – Ray Tube]:

Mode of operation:

 A low voltage is applied across the cathode and electrons are emitted thermionically and a concave
focusing cathode focuses the electrons onto the target.

 The high potential difference applied across the cathode and the anode accelerates electrons across
the vacuum and on reaching the target, 99% of the kinetic energy of electrons is converted into heat
while 1% of kinetic energy of elections is turned into X–rays.

 The heat generated at the target is cooled by means of copper cooling fins or running water and then
conducted away by conduction and radiation.

 The X – ray tube is evacuated to prevent cathode rays from colliding with air particles [air resistance]
hence allowing free movement of electrons in the tube.

 The tungsten is used because it has a high melting point that can withstand the heated generated
when electrons hit the target.

 In the X – ray tube the following energy changes take place;

Electrical energy Heat energy Kinetic energy Electromagnetic energy

Intensity of X- Rays [Quantity]:

Intensity is the strength or power of X – rays. The intensity of X – rays in an X – ray tube is proportional
to the number of electrons reaching the target. The number of electrons produced is determined by the
filament current. Therefore the higher the filament current the higher the intensity of the X – rays since
more electrons are emitted with high filament current.

Penetration power of X – Rays [Quality]:

Penetration power is the ability to enter matter. The penetration power of X – Rays depends on the
kinetic energy of electrons reaching the target. The penetration power of X – Rays is determined by the
high potential difference across the X – Ray tube. The higher the accelerating voltage the faster the
electrons produced and the greater the kinetic energy of electrons hence the higher the penetration
power of X–Rays produced.
Types of X- Rays:

There are two types of X – rays and these are

i) Soft X – rays.
ii) Hard X – rays.

Soft X– Rays:

These are types of X – rays produced when a low potential difference is used.

Properties of soft X – Rays:

 They produced by low voltages.

 They have low penetration power.

 They have low energy.

 They have long wave length.

 They are used in X – ray photography for human body.

Hard X – Rays:

These are types of X – rays produced when a high potential difference is used.

Properties of hard X – rays:

 They produced by high voltages

 They have high penetration power.

 They have high energy.

 They have short wave length.

 They are used to destroy cancer cells.

Properties of X – rays:

 They carry no charge.

 They are not deflected by both magnetic and electric fields.

 They readily penetrate matter. Penetration is least with materials of high density

 They cause ionization of gases.

 They affect photographic paper.

 They travel in straight lines at a speed of light.

 They undergo reflection, refraction and diffraction by atoms.

 They are electromagnetic waves of very short wave length.

 They cause other materials to give off light [fluorescence].

 They can produce photoelectric emission

Uses of x – rays:

Uses of X-Rays can be characterized as medical uses and industrial uses Medical uses:

 They are used to investigate the broken bones in X – ray photography.

 They are used to treat cancerous diseases and other malignant growth in the human

 They used to detect the complicated organs of the body.

 They are used to detect tuberculosis of the lungs.

 They are used to diagnose stomach ulcers

Industrial uses:

 They are used to detect cracks in metal castings and welded joints.

 They are used to study the structure of crystals [crystallography].

 They are used to detect faults in motor tyres.

 They are used to detect defects in paints.

Safety precautions taken when using x – rays:

 A void unnecessary exposure to X – rays.

 The X – ray apparatus should shielded using thick lead.

 The person should wear protective clothing made of thick lead.

 Keep large distance between X-Ray source and people.

 Soft X – rays should always be used on human tissues

Dangers of x – rays:

 They destroy living cells in the body.

 They damage blood cells and eye sight.

 They cause genetic changes [mutation].

 They can cause deep seated burns due to their greater penetration power.
NUCLEAR PROCESSES

By the end of this chapter, learners will be able to:

a) Understand the processes of nuclear fission and fusion and the associated energy changes.
b) Understand the spontaneous and random nature of nuclear decay and interpret decay data in
terms of half-life.
c) Know the applications of radioactivity and the dangers associated with exposure to
radioactive materials.
d) Understand and appreciate that there are significant social, political and environmental
dimensions associated with use of nuclear power.

Definition:

This is a process in which energy is produced. Nuclear processes, includes; fusion, fission, and
radioactive decays of unstable binding energies.

NB:

1. the total number of neutrons plus protons does not change in any nuclear process
2. Strong and weak nuclear interactions determine nuclear stability and processes.

There are two types of nuclear reactions and these are

i) Nuclear fusion.
ii) Nuclear fission.

Nuclear fusion:

This is a process by which two light nuclei combine to form a heavy nucleus with release of energy. It
takes place at the sun, stars and in the hydrogen bomb. Example Two Deuterium nuclei combine to form
Helium -3(tritium) and a neutron with release of energy

Conditions for nuclear fusion:

i) It occurs at very high temperature of about 108K.

ii) Presence of two light nuclei.
iii) It requires high speed moving nuclei.

Nuclear fission:

This is the splitting of a heavy nucleus into two nuclei with release of energy. It takes place in nuclear
reactors and in the atomic bombs.

Example

Splitting of uranium-236 to form Barium (Ba) and Krypton (Kr) with release of energy
Conditions for nuclear fission:

i) It occurs at very low temperature.

ii) Presence of energetic slow moving neutron.
iii) Presence of a heavy nucleus,

Examples:

Find the values of x and y in the above nuclear fission reaction

236 = 144 + y + 2 92 = x + 36 + 0

236 = 146 + y 92 = x + 36

y = 90 x = 56

Identify X in the above nuclear fission reaction

235 + 1 = 144 + 90 + 2A 92 + 0 = 56 + 36 + 2z

236 = 234 + 2A 92 = 92 + 2z

2 = 2A 0 = 2z

A=1 z=0

X is a neutron

NB: The number of radioactive nuclei that decay per unit time is called activity

NUCLEAR REACTOR:

The diagrams below shows a nuclear reactor that produces electricity by nuclear fission;
How electricity is produced from the nuclear reactor:

The uranium rod undergoes fission and fast neutrons are produced as a result. The neutrons collide with
other atoms and the energy possessed by the neutrons changes to heat energy. The heat energy is then
absorbed by the coolant and is made to heat water to produce steam. The steam then drives the
turbines connected to a generator and electricity is produced.

Note: that in thermal power plants, the fossil fuels burn coal, oil or natural gas to generate heat. In a
nuclear energy plant, heat is produced by a chain reaction of nuclear fissions.

Social, political and environmental issues associated with use of nuclear power

Economic development and population increase are boosting a new process of energy demand all
around the world. Whereas fossil fuels represent the cheapest sources of energy however they are big
environment polluters. Nuclear energy fulfils three of the main objectives that should be pursued for a
steady development:

i) It does not emit greenhouse gases

ii) It is the cheapest produced energy
iii) It guarantees a security in its supply due to the fact that it is not conditioned by external factors.

Nuclear and radiation accidents and incidents

Remember the body that regulates the use of nuclear energy is called the international atomic energy
agency (IAEA). IAEA defines nuclear and radiation accident as “a nuclear event that has led to significant
consequences to people, the environment or the facility. Examples includes; lethal effects to individuals,
large radioactivity release to the environment, reactor core melt.

Task: Research on internet and give me your findings on;

a) Cases of nuclear and radiation accidents in the recent past

b) The consequences of the nuclear and radiation accidents to the people and environment.

Functions of the parts:

i) Control rods are made of boron or cadmium and they control the fission rate that is they
absorb neutrons that would initiate a fission reaction.
 ii) The moderator is made of graphite or heavy water at very high pressure and it is used to
slow the neutrons there by making them to be absorbed by the uranium atoms
iii) Coolant is made of water at very high pressure, liquid sodium or carbon-dioxide and is used
to absorb heat from the reactor core.

iv) Uranium rods are made of uranium and they are as fuel which contains uranium-235. The atoms
of uranium undergo fission and produce energy

iv) The concrete shield absorbs any radiation from the fission fragments or from the fuel
directly.

NOTE:

Heavy water at very high pressure is used instead of ordinary water as a moderator because heavy

water does not absorb the neutrons since it contains deuterium atoms and high pressure
prevents it from boiling and high pressure prevents it from boiling and thus not turning into a vapor.

Uses of nuclear reactors:

i) They used to produce new elements.

ii) They are used to produce nuclear fuel.
iii) They are used to produce power that it acts as source of energy.

Problems nuclear power station face:

i) There may be an explosion due the radiations from the environment.

ii) There may be fuel leakage.
iii) Global warming due to the heat given out.
iv) Dangers from radioactive waste.

RADIOACTIVITY:
Many significant activities in nature take place around us and are unnoticed. Emission and
absorption are some of the phenomena that go unnoticed when it takes place in the atom.
Radioactivity is one such property of the matter where the emission of energetic sub-atomic particles
take place spontaneously. In this concept, we will learn more about radioactivity.

Definition:
This is the spontaneous (uncontrolled) disintegration (separation) of unstable nucleus to form a
stable nucleus with emission of radiations. Radioactivity is not affected by eternal factors like
temperature and pressure and it mainly occurs due to unstable nature of some isotopes which will
make them decay as a result in order to reduce the size or become more stable.

Products of a radioactive decay

There are three radiations emitted by radioactive nucleus and these are;

i) Alpha particles, α.( emission consists of helium nucleus)

ii) Beta particles, β.(emission consists of electrons)
iii) Gamma rays, γ ( photons having high energy emitted)

Investigating the characteristics of the radioactivity radiation

Key question: describe the characteristics of the radioactivity radiations

Alpha particle, α:

Alpha particle is a high speed helium nucleus

Alpha particles have a mass number of 4 and atomic number of 2 and carry a positive charge.
Properties of alpha particles:

 They are slightly deflected by both magnetic and electric fields because of their large mass.

 They are positively charged.

 They have the greatest ionizing power.

 They have the least penetrating power.

 They are stopped by a thick sheet of paper.

 They have very short range in air.

 They affect the photographic films.

 They have speed less than the speed of light.

 They produce flashes when incident on fluorescent substance.

Beta particle, β:

A beta particle is a high speed electron emitted from the nucleus of a radioactive atom. Beta
particles have no mass number and carry a negative charge . A beta particle is produced as a
result of one of the neutrons changing to a proton

Properties of beta particles:

 They are negatively charged.

 They are easily deflected by both magnetic and electric fields because they are lighter.

 They have greater penetrating power than alpha particles because of their high speed.

 They have less ionizing power than alpha particle.

 They can be stopped by a thin sheet of aluminum.

 They have a greater range in air than alpha particles.

 They produce flashes when incident on fluorescent substance

Gamma rays, γ:
Gamma ray is high energy electromagnetic radiation of very short wave length emitted from the
nucleus of the radioactive substance. Gamma rays have no mass number and carry no charge.
Gamma rays are produced when an excited atomic nucleus loses energy and the energy is given out
as gamma rays.

Properties of gamma rays:

 They are not charged.

 They travel at a speed of light since they are electromagnetic radiations.

 They are not deflected by both magnetic and electric fields since they are not charged.

 They have the least ionizing power.

 They have the greatest penetrating power.

 They undergo interference and diffraction.

 They cause fluorescence when incident on fluorescence substances.

Penetrating power of the radiations:

Alpha particles have the least penetrating power and can be stopped by a thick sheet of paper. Beta
particles have greater penetrating power than alpha particles and can be stopped by a thin sheet of
aluminum while gamma rays have the greatest penetrating power and can be stopped by thick block
of lead

Ionizing power of the radiations:

Alpha particles produce straight traces because they are heavy and they cause greater ionization of
the gases through which they pass. Beta particles produce irregular and light traces while gamma
rays do not produce any trace when the radiations are in a cloud chamber detector.

Deflection of the radiations in an electric field:

When the radiations from a radioactive nucleus are passed through a strong electric field the beta
particle s are deflected towards a positive plate showing that they carry a negative charge. Alpha
particles are deflected towards a negative plate in the direction opposite to that of beta particles
showing that alpha particles carry a positive plate. The gamma rays are not deflected at all showing
that they carry no charge.

Deflection of the radiations in a magnetic field:

When the radiations from a radioactive nucleus are passed through a strong magnetic field the beta
particles are deflected according to Fleming’s right hand rule showing that they carry a negative
charge. Alpha particles are deflected to the direction opposite to that of beta particles showing that
alpha particles carry a positive charge but heavier than beta particles. The gamma rays are not
deflected at all showing that they carry no charge.

Radioactive decay:

This is the process of spontaneous break down of radioactive nuclide. A radioactive nuclide is an
atomic species of a radioactive substance which continuously breaks down with emission of
radiations.

Types of radioactive decay:

a) Alpha decay: When a nuclide undergoes an alpha decay, it loses two protons and two neutrons.
Therefore its mass number reduces by four and its atomic number reduces by two and the
daughter nuclide is two steps to the left in the periodic table.
Given that a radioactive element, ZAX undergoes an alpha decay to form element Y. Then the
nuclear reaction equation is given by

Parent nuclide daughter nuclide + product

Examples: 1.
Radium [Ra] decays to become radon [Rn] according to the equation

Uranium [U] decays to become thorium [Th] according to the equation

 Note: the helium nucleus resulting from alpha decay is called the daughter nuclei
Task:
a) Uranium-238 undergoes an alpha decay. Write down its decay equation and identify
the daughter nuclide.
b) If the under-listed nuclides undergo alpha decay, write down the decay equations and
identify

c) Beta decay:
When a nuclide undergoes beta decay its mass number does not change but its atomic
number increases by one and the daughter nuclide is one step to the right in the
periodic table. This is because electrons do not exist inside the nucleus but can be
produced when a neutron changes into a proton and an electron, the proton then
remains in the nucleus hence increasing the atomic number and an electron is lost from
the nucleus.
Given that a radioactive element, ZAA undergoes a beta decay to form element Y. Then
the nuclear reaction given by

Task:

a) Thorium-234 undergoes a beta decay. Write down its decay equation and
identify the daughter nuclide.
b) If the under-listed nuclides undergo beta decay, write down the decay
equations and identify the daughter nuclides. Hint: you may need a periodic
table
1. Zinc-65
2. Iodine-127
3. Tungsten-184
c) Gamma decay:
Gamma rays are not particles, therefore when nuclide emits gamma rays its
atomic number and its mass number do not change but the nucleus becomes
more stable.
Given that a radioactive element, ZAA undergoes a gamma decay to form a
stable element X. Then the nuclear reaction equation is given by

Half-life of a radioactive mater:

This is the time taken for a radioactive element to decay to half its original value. Half-life is measured in
seconds, minutes, hours, days, weeks, months and years. Half-life is not affected by physical factors like
temperature and pressure and half-life is different for different radioactive nuclides.
A block of radioactive material will contain many trillions of nuclei and not all nuclei are likely to decay
at the same time since the decay is spontaneous. It is not possible to say which particular nucleus will
decay next. But given that there are so many of them, it is possible to say that a certain number will
decay in a certain time. Although scientists cannot tell when half the unstable nuclei in a sample will
have decayed.

Note: that the decay process is exponential and can be represented by the graph shown below. And
exponential decay means that the higher the number of undecayed nuclides in the sample, the greater
the rate of decay of the sample.

Understanding the decay curve:

Task:

a) Go to your learner’s guide and attempt example 6.3 on page 82

b) Go to your learner’s guide and attempt exercise 6.3 on page 83

Uses and dangers of radioactivity

Note: The change of an element to another element is called Transmutation

Uses of radioactivity:

a) Medical uses:

 Radiations are used in radiotherapy [in treatment of cancerous cells] that is gamma rays are
usually used to destroy cancer cells.

 Medical instruments are sterilized using gamma rays.

 Radiations are used to detect brain tumors.

 Radiations are used to detect lung and heart problems.

 Radiations destroy germs.

b) Industrial uses:

 Radioactive elements are used to measure fluid flow in pipes in industries.

 Radioactive elements are used to provide source of energy [electricity].

 Radioactive elements are used in hardening polythene and petroleum.

 Radioactive elements are used in food preservation.

 Radioactive elements are used as tracers in identifying oil leakages in oil pipes.

 Radioactive elements are used to measure the thickness of the metal sheet.

 Radioactive elements are used as level indictor that is to check the filling packets of soap
powders.

c) Agricultural uses:

 Radioactive elements produce varieties of plants with new characteristics.

 Radioactive elements are used as tracers to study the uptake of fertilizers by plants.

 Radioactive elements can be used in pest control.

d) Carbon dating:

 Carbon dating forms a radioactive carbon-dioxide which is taken up by plants in the manufacture
of carbohydrates by photosynthesis. When plants are cut down the atoms will start to decay by
emission of beta particles and by measuring the residue and half-life the age of the ancient
containing carbon can be estimated.

Health hazards of radioactivity:

 Radiations cause blood cancer.

 Radiations cause radiation burns.

 Radiations cause sterility [inability to produce]

 Radiations cause low body resistance to normal diseases.

 Radiations cause genetic changes [mutation].

 Radiations destroy body cells.

 Radiations damage eye sight and body tissues.

Safety precautions when handling radioactive elements:

 They should be handled using long pair of tongs.

 They should be transported in thick lead containers.

 You should avoid unnecessary exposure to the radiations.

 You should wear protective clothing when handling radioactive elements.

 You should not eat or drink where radioactive sources are in use.

 You should cover any wound before using radioactive source.