Radioactivity

Introduction
 Reactions involving the electrons are called chemical reactions.
 Reactions originating from the nucleus are referred to as nuclear reactions.
Stability of isotopes of elements
 Nucleus of an atom characterised by protons and neutrons is known as nuclide.
 A nuclide is represented by its mass number and atomic number as shown 35 17𝐶𝑙
 The superscript represents mass number (sum of protons and neutrons in the nucleus) while
the subscript represents atomic number (protons).
 Isotopes are atoms of the same element with the same atomic number but different mass
number.
 Nuclides can either be stable or unstable.
 Most stable nuclides have a neutron: proton (n/p) ratio of about 1:1 e.g. 40
20𝐶𝑎 has 20 neutrons
and 20 protons hence n:p =20:20 =1:1
 For nuclides of high atomic number, the n/p ratio increase progressively up to about 1.6:1
which is still within the stability region.
 Above this value the nucleus becomes too large and unstable.
 The unstable nuclide normally undergoes some changes by emitting radiation in form of
particles and energy.
 Such emissions results in production of new nuclides of completely different composition.
What is radioactivity?
 Is the process where an unstable nuclide breaks up to yield another nuclide of different
composition with emission of particles and energy.
What are radioactive substances?
 Are substances which undergo radioactivity
What are radioisotopes?
 Are isotopes that undergo radioactivity. Also called radioactive isotopes.
What is radioactive decay?
 It is the spontaneous disintegration of radioactive nuclides.
N/B - Radioactivity is a nuclear process and not a chemical process.

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Differentiate between chemical reactions and nuclear reactions
Nuclear reaction Chemical reaction
 Involves neutrons and protons in  Involves electrons in the outermost
the nucleus energy level (valence electrons)
 Large amounts of energy change.  Little amount of energy change.
 New elements are formed.  No new elements are formed.
 The rate of reaction is not affected  Rate of reaction is affected by
by external factors. external factors such as
concentration temperature and
pressure.
 Changes are irreversible  Some changes are reversible.

List the two types of radioactivity

 Natural radioactivity
 Artificial radioactivity.
Differentiate between the two types of radioactivity mentioned above.
 Natural radioactivity is a radioactive process where a radioactive (unstable) nuclei split
spontaneously giving a new nuclide with emission of radiation and lots of energy.
 E.g. nucleus of uranium -238 undergoes natural radioactive decay to form Thorium -234 and
some radiation.
 Artificial radioactivity is a radioactive process where a large stable nuclides are bombarded
with fast moving high energy particles, where they split into relatively smaller nuclei with
emission of radiation and lots of energy.
Name three types of nuclear radiations.
 Alpha particles( 𝛼)
 Beta particles( 𝛽)
 Gamma rays(𝛾)
Characteristics of radiations
Alpha particles
 Alpha radiations are positively charged helium nuclei, He 2+ and are represented by 42𝐻 2+ in
nuclear equations.
 Alpha particles have an electrical charge of 2+.
 They are the heaviest of the three.
 Recall that helium atom has two electrons in its one energy level and that it can lose the two
electrons to remain with nucleus only which has 2 neutrons and 2 protons and with a charge
of 2+.
 The mass of helium nucleus is 4 i.e (2 protons +2 neutrons = mass number).The helium
nucleus therefore is similar to 𝛼 –particle.

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 Beta particles
 Beta radiations are particles which are negatively charged.
 Are represented by − 10𝑒 .
 They are electrons which originate from within the nucleus and not from the outer energy
levels.
 They are formed when a neutron changes into a proton within the nucleus.
1 1 0
0𝑛 → 1𝑝 + −1𝑒

(Neutron) (Proton) (Electron)

 The -1 is not an atomic number .it represents the charge on the particles .beta particles have
an electrical charge of -1.
Gamma rays
 Gamma radiations are high energy rays.
 They do not have an electrical charge.
 They are not emitted on their own but normally accompany the emission of alpha and beta
particles.
Comparison of the properties of the three types of nuclear radiations
Property Alpha particles( 𝜶) Beta particles( 𝜷) Gamma rays(𝜸)
Charge  Are positively  Are negatively  Are neutral(no charge
charged particles. charged or mass)
Deflection by  Are deflected to  Are deflected to a  Not deflected by an
an electric negative terminal of positive terminal electric field.
field electric fields(are of electric field
attracted to negative (attracted to
plate) positive plate)
-deflection is smaller -the deflection is
due to their heavy greater due to their
masses. much smaller
masses
Ionizing  They have high  Have lower  Have very low
effect ionizing power. ionizing power and ionizing power.
(power)  They produce large produce fewer ions
number of ions as in gases
they pass through
gases due to their
slow speed and high
charge.
 Their slow speed
enables them to be in
contact with target
atoms for longer time.
Penetrating  Have very low  Have higher  Have the highest
power penetrating power. penetrating power. penetrating power of
 They do not pass a  Can pass through a the three.
sheet of paper sheet of paper but  They pass through a
are stopped by a sheet of paper and a
sheet of aluminium foil of aluminium
foil. but will be stopped
by a thick lead block

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Diagrams showing the a) penetrating power of the radiations and b) deflection of radiations by an
electric field .
a)

b)

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Radioactive Decay and Half –Life
 When a radioactive nuclide decays, a new nuclide is formed.
 As disintegration proceeds, fewer and fewer unstable atoms remain.
 The table below shows the radioactive decay for a sample of 400g of iodine – 131
Time (Days) Mass (g) of radioactive iodine remaining
0 400
8.1 200
16.2 100
24.3 50
32.4 25

Questions
1) Plot a graph of mass (g) of iodine remaining (y-axis) against time (days)(x-axis)?

2) How long does it take for the amount to be reduced to half?

 It takes 8.1 days for the amount of substance remaining to be half the previous
amount.
3) What fraction of the original amount remains after (i) 8.1 (ii) 16.2 (iii) 24.3?
i) 8.1 days is ½
ii) 16.2 days is ¼
iii) 24.3 days is 1/8
4) If the sample continues to decay predict how long it will take the remaining amount to
reduce to zero
 The remaining amount will never reduce to zero.

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Half- life
 Earlier we have learnt that radioactive isotopes decay emitting either 𝛼 –
alpha particles , 𝛽- beta particles or 𝛾-rays.
 The decay occurs in different rates , where some take few microseconds, some
minutes, hours, days or others may take thousands of years to decay.
 The rate of decay is a characteristic of the isotope and it is measured by its
half life, (normally expressed as t ½)
 After one half life, half of the original radioactive atoms would have decayed.
 After a second half life, a quarter will be remaining, and tis goes on and on.

Half life is therefore defined as follows

 Half - life of a radioactive isotope is the time taken for a given mass or
number of nuclides to decay to half its original mass or number.

 From the sample given above (400 g of iodine -131 ) the amount remaining
after the first half life (8.1) will be 200g.After the second half –life (16.2days)it
will be 100g.
 This can be illustrated as shown below.
𝐼𝑠𝑡 ℎ𝑎𝑙𝑓 𝑙𝑖𝑓𝑒 2𝑛𝑑 ℎ𝑎𝑙𝑓 𝑙𝑖𝑓𝑒 3𝑟𝑑 ℎ𝑎𝑙𝑓 𝑙𝑖𝑓𝑒
8.1 𝑑𝑎𝑦𝑠 8.1 𝑑𝑎𝑦𝑠 8.1 𝑑𝑎𝑦𝑠
 400g → 200g → 100g → 50g
 The amount remaining after the first half- life is half the previous amount.
 The remaining amount after successive nuclide decay can be worked out by
using the formula below:
Remaining amount = (½ )n x original amount
Where n is the number of half –lives undergone.
For example the remaining amount after 24.3 days (3 half-lives) is.
(½ )3 x 400 =1/8 x 400 =50g
 Also when given the remaining amount , the original amount can also be
determined.
Question
If 3g of 257
103𝐿𝑟 whose half life is 8 seconds remain after undergoing
radioactive decay for 32 seconds, calculate the original amount.
Solution
32
The number of half life = 8 = 4
Using the formula (½ )4 x Original amount =3
32
Original amount x 8 = 3
= 48g

Alternatively , the original amount can be found using a step by step method
as shown below
4𝑡ℎ ℎ𝑎𝑙𝑓 𝑙𝑖𝑓𝑒 3𝑟𝑑 ℎ𝑎𝑙𝑓 𝑙𝑖𝑓𝑒 2𝑛𝑑 ℎ𝑎𝑙𝑓 𝑙𝑖𝑓𝑒 1𝑠𝑡 ℎ𝑎𝑙𝑓 𝑙𝑖𝑓𝑒
8 𝑠𝑒𝑐 8𝑠𝑒𝑐 8𝑠𝑒𝑐 8𝑠𝑒𝑐
3g → 6g → 12g → 24g→ 48g

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 Some radioactive isotopes and their half life are listed below.
Nuclide Half-life

212
84
Po 2.96 × 10 -7 seconds

257
103
Lr 8 seconds

234
91
Pa 1.14 minutes

234
90
Th 24.5 days

1
1
H 12.3 years

14
6
C 5570 years

234
92
Po 4.5 ×10 9 years

 A shorter half- life means a faster rate of decay of a nuclide.

 On the other hand a longer half –life means a slower rate of decay.
 As noted earlier, radioactive decay is never affected by any chemical or physical change.

Nuclear reactions (changes in nuclei due to radioactive decay)

 During decay, radioisotopes emit radiations in form of 𝛼 –alpha particles , 𝛽- beta particles
or 𝛾-rays.
 When this happens new nuclide(s) is/ are formed(in otherwords , new elements are formed)
a) 𝛼 -particle decay
 Alpha particle 42𝐻 2+ consists of two protons and 2 neutrons.
 An element that undergoes 𝛼 – decay therefore has its mass(A) reduced by a value of
four (4) and its atomic number(Z) reduced by two (2).
Example
𝐴
𝑍
X → (𝐴 − 4)
(𝑍 − 2)
Y + 42𝐻2+
(parent nuclide) (new nuclide) (𝛼 – particle /helium nucleus)

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Other equations for specific examples of this type of decay.

210 206 4 2+
1.
84
Po → 82
Pb + 2𝐻

Polonium-210 Lead-206 𝛼 – particle

226 222 4 2+
2.
88
Ra → 86
Rd + 2𝐻

Radium-226 Radon-222 𝛼 – particle

238 234 4 2+
3.
92
U → 90
Th + 2𝐻

Uranium- 238 Thorium-234 𝛼 – particle

N/B
 If more than one 𝛼 – particle are emitted in decay process,for example 4 𝛼 –
particles, then the atomic number and mass number of helium nucleus/ 𝛼 –
particle is multiplied by four and then subtract the resultant figures from the
mass number and atomic number of the parent nuclide as shown below
 𝐴𝑍 X → (𝐴 − (4 × 4))
(𝑍 − (2 × 4))
Y + 4 42𝐻2+

b) 𝛽- particle decay .
 𝛽- beta particle is an electron ( −10𝑒)
 When an element decays by emission of a beta particle,it turns into the next element in
the periodic table, since atomic number increases by 1.
 There is no change in mass number.
 The change that occurs is as shown in the equation below.

𝐴 𝐴 0
𝑍
X → (𝑍 − (−1))
Y + −1𝑒

Nuclear reactions for specific examples of beta decay.

234 234 0
1.
90
Th → 91
Pa + −1𝑒

Thorium-234 Protactinium-234 𝛽– particle

23 23 0
2.
11
Na → 12
Mg + −1𝑒

Sodium-23 Magnesium-23 𝛽– particle

N/B- If more than one 𝛽– particle is emitted,then you multiply( −10𝑒 )by the
number of particles emited.
Take an example where 23490
X loses three 𝛽– particles,then;

234 − 0
 234
90
X → 90 − (−1 × 3)
Y + 3 −10𝑒 Hence

 234
90
X → 234
94
Y + 3 −10𝑒
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 c) 𝛾-radiation

 Have no charge and mass.

 Their emission therefore have no effect on the mass and atomic numbers of the resultant
daughter nuclide.

Radioactive decay series

 Radioisotopes can undergo natural continuous decay to form various unstable isotopes.
 This continues until a stable nuclide is formed.
 In the process of decay, 𝛽– particle and 𝛼 –particles are emitted.
 Radioactive decay series can therefore be defined as a sequential and continuous
disintegration of unstable nuclide until a stable nuclide is formed.

Radioactive decay series of thorium-232 (Thorium decay series)

o Ends when a stable nuclide (lead-208 )is formed.

Nuclear fission and nuclear fusion

Nuclear fission
 Radioactive nuclides split naturally to emit 𝛽– particle and 𝛼 –particles and 𝛾-rays.
 The splitting can however be initiated by bombarding a heavy unstable nuclide with fast
moving particles such as neutrons.
 Nuclear fission is the splitting of a heavy a heavy unstable nucleus when bombarded by a fast
moving neutron.

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 Example –Nuclear fission of U-234
234
92
U + 10𝑛 → 90 38
Sr + 144
54
Xe + 1
0𝑛 +energy
 It should be noted that the total sum of the superscripts on the left and right sides of the arrow
should be equal, and the same applies to the subscripts.
 Nuclear fission can also result from fast moving alpha particles as well.
 N/B- Fission unleashes enormous amounts of energy.For example,1kg of uranium-235
releases an amount of energy equal to that generated in an explosion of 20,000 tons of
dynamite.

Nuclear fission (adapted from longhorn chemistry book 4)

Nuclear fusion
 It is the process where light nuclei combine together when they are made to collide at high
velocity.
 The process is accompanied by liberation of large amounts of energy.
 Sub-atomic particles such as neutrons are also released during nuclear fusion.

Examples of nuclear fusion

𝒇𝒖𝒔𝒊𝒐𝒏
2 1 3
1.
1
H + 1
H → 2
He+ energy

𝒇𝒖𝒔𝒊𝒐𝒏
2 2 4 1
2.
1
H + 1
H → 2
He + 0
n + energy

𝒇𝒖𝒔𝒊𝒐𝒏
2 4 0
3. 4 1
H → 2
He + 2 −1 n + energy

 The energy released during nuclear fission and nuclear fissioncan be harnessed and
converted into other forms of energy.

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 Similarities between nuclear fission and nuclear fusion
1. Large quantities of energy is released in both cases.
2. Both processes result into chain reactions.
3. Sub-atomic partciles like neutrons accompany both processes.
4. Energy released can be harnessed and converted into other forms of useful energy.
5. The large quantities of energy produced can be highly destructive when misused
e.g in nuclear warfare.
Applications of radioactivity
In medicine.
a) Penetrating radiation like 𝛾 − 𝑟𝑎𝑦𝑠 can be used in the treatment of malignant tumours and
cancers (Side effect- prolonged exposure may lead to damage of other healthy tissues)
b) Radioactive iodine-131 is used to study working of the thyroid gland.
c) Sterilization of surgical instruments using gamma rays.
d) For providing power in heart pace setters.
e) Used to monitor growth of bones and healing of fractures. (X-rays)

In agriculture.
f) Monitoring photosynthesis and related processes.
g) Determining the rate of absorption of a fertilizer.(using radioactive phosphorous)

Others
h) Carbon-14 dating to determine the age of archeological materials in fossils.
i) Gauging the thickness of thin metal and paper sheets in manufacturing industries.
j) Preservation of food by exposing micro-organisms to gamma radiation to kill them.
k) Measuring the level of food in canned and packed food.
l) Manufacture of nuclear weapons and atomic bombs.

Pollution from Radiactivity

 𝛽– particle and 𝛼 –particles and 𝛾-rays end up in air , water and soil.
 These get exposed to man through the air breathed,water or food crops taken which
were grown from contaminated soil.

Dangers of 𝜶 –radiation
 Not able to penetrate skin.
 Enter the body when materials emitting 𝜶 –radiations are swallowed, inhaled or get
through open wounds.
 Cause damage to cells and body organs.

Dangers of 𝜷– radiations
 When materials emitting these are left on skin for some time they cause skin injury and
may interfere with the skins ability to produce new cells.
 Such materials may also get into the body through swallowing , inhaling or open wounds,
hence causing damage of cells and organs.

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 𝑫𝒂𝒏𝒈𝒆𝒓𝒔 𝒐𝒇 𝜸-rays
 Are known as penetrating radiations because they can readily penetrate human tissue.
 They lead to damage of body tissue and organs.
 Can also lead to mutations which can cause permanent change of body structure.
 May damage the DNA molecules in cell nuclei which may later carry faulty genetic
information in future generations.

Protection against Radiations

 Do not handle radioactive materials unnecessarily.
 Use correct handling equipment eg tongs.
 Keep distance when working with radiactive materials.
 Use the shortest time possible when working with such materials to reduce the exposure.
 Use shields (made of special glass or lead block)to protect yourself.

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