Chemistry

Key unit competence:

To be able to explain the importance and dangers of radioisotopes in everyday life.

Learning objectives:

• Explain the process of radioactivity.

• Explain the properties of alpha, Beta and Gamma rays.

• Develop awareness of the dangers of radioactive substances and nuclear

weapons.

• Write and balance nuclear reaction equations.

• Compare and contrast chemical and nuclear reactions.

• Explain half-lives of radioactive isotopes/radioisotopes.

• Perform calculations involving the half- life of radioactive substances.

• Explain the applications of radioisotopes in medicine, agriculture and industries.

• Apply the calculations of half-life to determine the age of fossils.

Introductory activity:

1. Have you ever heard about Radioactivity?

2. If yes, can you explain what it is?

3. Can you mention some applications of Radioactivity?

4. What is the difference between an ordinary chemical reaction and a

nuclear reaction?

You are provided with illustrations which are linked with radioactivity. Observe

them and try to analyse them.

The above photos show different ways through which radiations reach into

our body (cells). The most familiar to you is radiation from sun rays! However,

everything present in the pictures above, and many others that are not included,

emits radiation. Now, discuss on the following points. Note: If you get stuck by

some points, don’t be frustrated! You are allowed to visit any important document

(textbook, search engine …) that can help you to find out the

required solution.

Point 1: Can you see or feel the presence of radiation?

Point 2: How are we exposed to natural ionizing radiation? (Radiation with the

ability to rip out one or several electrons from an atom or molecule is referred

to as ionizing radiation and Radiation which does not have sufficient energy to

damage atoms or molecules is called non-ionizing radiation).

Point 3: What do you understand by “radioactive materials”? Do you think

all of them are natural? Do you think all of them (i.e, radioactive materials) as

harmful?

The discovery of the electron towards the end of the nineteenth century was the

starting point of new avenues of research in science, which were to give physicists

an insight into the structure and nature of the atoms of matter. They discovered

that there is a nuclear phenomenon in some elements that pushes them to emit

radiations.

This phenomenon was called radioactivity as proposed by Marie Curie to describe

those emissions of nuclear radiation by some of the heavy elements.

Radioactivity is an integral part of our environment. All living beings have been

exposed to a constant flux of natural radiation on the surface of our friendly planet,

but this radiation has no negative effect.

Particles and rays are emitted when a nucleus of a radioactive isotope of element

breaks down. They break down to acquire stability as all atoms want to be stable.

We are constantly being bombarded by particles of cosmic radiations: several

hundred go through our bodies every second. Rocks like granite, which have become

symbols of permanence and durability (hence used in building), contain light traces

of radioactive uranium. Sitting on or walking near a block of granite exposes you to

many sources of radioactivity. Even the food we eat or the air we breathe contains

radioactive elements (such as radon) either formed by the intervention of cosmic

rays, or as old as the solar system itself. There is absolutely no way to escape from

it: we are even radioactive! Eight thousand atoms of potassium-40 or carbon-14

disintegrate in our bodies every second.

15.1 Definition of radioactivity, radioisotopes and comparison

between chemical and nuclear reactions.

Activity 15.1

1. What do you think about the terms ‘radioisotopes’ and ‘radioactivity’? How

can you define the two terms from your understanding? From chemical

and nuclear equations, describe any differences between these two types

of equations

2. Now use a search engine or textbooks (including even this one) to find

out appropriate explanations on radioisotopes, radioactivity and nuclear

equation, then make a good report to share with others.

15.1.1. Definition of radioactivity and radioisotopes

Radioactivity is a nuclear phenomenon. It is the process by which an unstable atomic

nucleus changes into another more stable atomic nucleus by emitting energy in form

of radiation. Substances which have the property of emission of radiation are called

radioactive substances. Radioactivity is also known as radioactive disintegration

or radioactive decay.

A radioactive decay results when an atom with one type of nucleus, called “the

parent radioactive nuclide” transforms into another atom with a different nucleus.

The new product or element is named “the daughter nuclide”;, and thus the decay

process results in “transmutation”. Transmutation, in this case, means creation of

an atom of a new element. In this way, the energy or radiation emitted may take the

form of particles such as alpha (α) or beta (β) particles.

During the transmutation process, daughter nuclides are often in metastable or

excited state; they lose energy in form of gamma ( γ ) ray to become de-excited.

Gamma rays, here, can be compared to the heat of reaction that accompanies an

exothermic reaction.

In nuclear chemistry, the term ‘nuclide’ is used to designate a nucleus of an element.

‘Nucleons’ is the term used for nuclear particles such as protons and neutrons. In

radiochemistry, the nucleon number stands for mass number (sum of protons and

neutrons present in the nucleus of a given atom).

Different nuclides, which have the same proton number but different nucleon

numbers are called isotopes or isotopic nuclides.

Radioisotopes or radioactive isotopes are the atoms of an element whose atomic

nuclei undergo decay by emitting radiation(s).

15.1.2. Comparison between chemical reaction and nuclear reaction

Nuclear reaction and chemical reaction differ as shown in the following table:

Table 15.1: Differences between a chemical reaction and a nuclear reaction

Checking up 15.1:

1. Explain what is meant by each of the following terms:

a. Radioactive decay.

b. Daughter nuclide.

c. Transmutation.

2. Differentiate between isotopes and radioactive isotopes

3. Describe the following as pertaining to chemical reaction or nuclear

reaction:

a. Isotopes have the same chemical properties as they have the same

number of electrons.

b. Hydrogen nuclei are the reactants.

c. Large amounts of energy are released.

d. The mass is strictly conserved.

15.2. Emission of alpha particles, beta particles and gamma

rays and their properties and effect of electric and magnetic

field on them.

Activity 15.2

1. State the types of radiation as revealed in the previous discussions.

2. Use your search engine or any textbooks available to find out the

properties the three types of radiation and the effect of electric and

magnetic fields on them.

15.2.1. Aplha particules, betaparticles gamma rays and their properties

Different forms of radiation are emitted from an unstable nucleus as it decays. The

main types of emitted particles are alpha particles, beta particles and gamma rays.

The detailed information on each particle is provided below.

a. Alpha partices

An alpha particle contains two protons and two neutrons (so, its mass number, A=4

and atomic number, Z=2). Because it has 2 protons, an alpha particle has a charge

of 2+ (+2). That makes it identical to helium nucleus. In equations, it is written as

the Greek letter “alpha (α)” or as the symbol for helium . The charge of an alpha

particle was found experimentally by passing it in an electric field between two

plates where it was attracted towards the negative plate.

The main properties of an alpha particle are the following:

• Alpha particle bears a positive charge of +2

• It has a mass of 4 amu

• It is deflected toward the negative pole of electric and magnetic fields. Look at

Figure 15.4 below.

• It affects a photographic plate and causes fluorescence on striking a fluorescent

material.

• It ionizes the gas through which it passes.

• Not very penetrating; a very thin sheet of aluminium foil or a sheet of paper

stops it.

• It can be shielded by paper or clothing.

• It destroys living cells and causes biological damage.

• It is strongly ionizing

The process of α-decay occurs commonly in nuclei with atomic number greater than

83. The nuclei of these elements are extremely unstable due to the large number of

neutrons and protons present. More information about stability of nuclei of atoms is

discussed in this unit, section 15.4.

When a nuclide decays by alpha emission, it loses 2 atomic number units and 4 mass

units; in other words the daughter nuclide is the element located at 2 places before

the parent nuclide in the periodic table.

This shows that the number of neutrons decreases by one and the number of protons increases by one. 

It is noteworthy that a β-emission results in the production of isobars (nuclides

having the same mass numbers but different atomic numbers). The beta emission

produces a new element with 1 more atomic number unit, or the element that

directly follows in the periodic table.

A positron is similar to an electron except that a positron has a positive (+1) charge.

A positron is produced by an unstable nucleus when a proton is transformed into a

neutron and a positron.

This shows that the number of neutrons increases by one and the number of protons

decreases by one; the element just preceding the parent nuclide in the periodic

table is formed.

Examples:

c. Gamma rays

Gamma rays, γ, are high-energy radiation released as an unstable nucleus undergoes

a rearrangement of its particles to give a more stable, lower energy nucleus.

Because gamma ray is an electromagnetic radiation, it has no mass and no charge.

The main properties of gamma radiation (gamma ray) are the following:

• It is an electromagnetic radiation of short wavelength and higher frequency,

hence high energy

• It is not deflected by electric and magnetic fields. Refer to figure 15.4 for more

understanding.

• It affects photographic plates.

• Its ionizing power is very low compared to alpha-particles and beta-particles.

• It travels at the same speed as that of light.

• It has the greatest penetrating ability, 5000-10000 times that of alpha particles.

• It causes fluorescence when they strike a fluorescent material.

• It is diffracted by crystals.

• It can be stopped by several inches (5cm thick piece) of lead or a thick concrete.

• It can easily pass through the human body and cause immense biological

damage.

Normally, there are very few pure gamma emitters. In radiology, one of most

commonly used gamma emitter is technetium (Tc). The excited state called

“metastable technetium” is written as technetium-99m, Tc-99m. By emitting energy

in the form of gamma rays, the excited nucleus becomes more stable.

Stable nuclides are usually in the state of least energy or ground state. But these

nuclides can be excited by particles or photon bombardment. The excited nucleus

returns into the ground state by emission of excess energy as ϒ-rays.

the properties of the particles discussed above.

Table 15.2: Distinction between the properties of α, β and γ radiations (summary)

Checking up 15.2

1. How are an alpha particle and a helium nucleus similar? Different?

2. Naturally occurring potassium consists of three isotopes: potassium-39,

potassium-40 and

Potassium-41.

a. Write the atomic symbols for each isotope.

b. In what ways are the isotopes similar and in what ways do they differ?

15.3 Nuclear equations and radioactive decay series

Activity 15.3

1. Use your search engine or any available resources to learn about nuclear

equations, how they are balanced and radioactive decay series and

make a summary to be presented. While carrying out your research try to

answer to question number 2 below.

More stable

A nuclear equation is balanced when the sum of the mass numbers and the sum

of the atomic numbers of the particles and atoms on one side of the equation are

equal to their counterparts on the other side.

The changes in mass and atomic numbers of an atom that emits a radioactive particle

are shown in the table below.

15.3.1. Nuclear equations

A nuclear reaction is represented by a nuclear equation as follows:

Example 1:

Radium-226 emits an alpha particle to form a new isotope whose mass number,

atomic number and identity we must determine.

Step 1: Write the incomplete nuclear equation.

Step 3: Determine the missing atomic number

Step 6: The equation is balanced.

Note: Nuclear reactions involving hitting a nuclide by a particle such as proton

or neutron as in the example 2 are referred to as ‘bombardment reactions’.

Bombardment reactions are not natural; they are induced (or artificial) nuclear

reactions.

For the case of example 1, when a radioactive particle is emitted, the type of nuclear

reaction is emission reaction; those are natural radioactive isotopes.

15.3.2. Radioactive decay series

Radioactive decay Series is the series of steps by which a radioactive nucleus

decays into a non-radioactive nucleus. The element goes from radioactive to nonradioactive.

A radioactive element disintegrates by emission of an α- or β-particle from the

nucleus to form a new “daughter element.” This again disintegrates to give another

“daughter element’. This is why the whole series of elements starting with the parent

radioactive element to the stable end-product is called radioactive disintegration

series or radioactive decay series as seen above.

Naturally radioactive nuclides disintegrate to acquire stability. In nature there are

four radioactive decays, that is, a series starting with a radioactive element and then

ending with a reasonable stable element.

The three series are Uranium, Thorium and Actinium series. Uranium series is the

most important.

The three series are similar because they all involve losses of alpha and beta particles

ending with isotopes of Lead. Uranium series gives Lead-206, the most stable isotope

of Lead; Thorium gives Lead-208 and actinium series gives Lead-207.

The fourth series is the Neptunium series which leads to Bismuth-209.

1. The uranium series

It starts with the parent element Uranium-238 and ends with the stable element

Lead-206. It derives its name from Uranium-238 which is the starting nuclide of the

series and has the longest half-life. In the process 8 alpha and 6 beta particles are

emitted before Lead-206 is attained. The whole process is shown belo

15.4. Stability and instability of nuclei of atoms

Activity 15.4

1. What do you think can influence the stability of the nucleus of a given

atom?

1. Use a search engine or any available resources to understand and share

everything about stability of nuclei of atoms.

Nuclides can either be stable or unstable toward radioactivity but a nucleus that is

unstable can become stable by undergoing a nuclear reaction or change.

Most isotopes of elements up to atomic number 19 have stable nuclei. Elements

with higher atomic numbers (20 to 83) consist of a mixture of isotopes, some may

have unstable nuclei. Elements with atomic numbers of 84 and higher consist only

of radioactive isotopes. So many protons and neutrons are crowded together in their

nuclei that the strong repulsions between the protons make these nuclei unstable.

Nuclear Stability is a concept that helps to identify the stability of an isotope. The

ratio of neutrons to protons (n/p) is a good indicator to know if an isotope is

radioactive or not.

To help you understand this concept, there is a chart of the nuclides known as a

Segre chart (see figure 15.5)

This chart shows a plot of the number of neutrons versus the number of protons

for known stable nuclides and shows the existence of the stability zone or band

of stability. It can be observed from the chart that there are more neutrons than

protons in stable nuclides with atomic number (Z) greater than 20 (Calcium).

These extra neutrons are necessary for stability of the heavier nuclei.

The excess neutrons act somewhat like nuclear glue. Atomic nuclei consist of protons

and neutrons, which attract each other through the nuclear force, while protons

repel each other via the electric force due to their positive charge. These two forces

compete, leading to stability of various nuclei.

Neutrons stabilize the nucleus, because they attract each other and protons, which

helps offset the electrical repulsion between protons. As a result, as the number

of protons increases, an increasing ratio of neutrons to protons is needed to form

a stable nucleus. If there are too many or too few neutrons for a given number of

protons, the resulting nucleus is not stable and it undergoes radioactive decay.

Checking up 15.4

1. What can you take as the basis to predict the stability of nuclei of atoms?

2. Use your periodic table to provide examples (2 in each case) of

a. Elements with naturally occurring stable isotopes.

b. Elements with naturally radioisotopes.

c. Elements with mixtures of both stable and unstable isotopes.

Please specify the atomic number of each element you have provided

15.5. Rate of decay of radioactive substances and half-life of a

radioisotope

15.5.1. Rate of decay of radioactive substances

As seen in units 13 and 14, the time for different chemical reactions to be completed

varies. In this unit, we are concerned with the calculation of decay rate for a decay

process which is a first order reaction.

Consider a simple case for a first daughter stable. Suppose that at time, t, there are

N radioactive nuclides and dN disintegrations in a time dt. The rate of disintegration,

dN/dt, is given by:

Worked examples

According to the radioactive decay law, the above information can be summarized

by plotting the graph iodine-131 percentages against time (half-lives)

From the decay curve, we can deduce a generalized formula that expresses how a

fraction of a nuclide decreases rapidly (exponentially) as time increases.

The half-life for a given isotope is always the same; it does not depend on how many

atoms you have or on how long they have been sitting around. Each radioactive

isotope decays at its own rate and therefore has its own half-life reason why it ranges

from fractions of seconds to millions (and even billions) of years. For example, look

at table 15.5.

• The shorter the half-life, the faster the decay of nuclide and the longer the

half-life, the slower the decay of the nuclide. Nuclides with short half-lives are

often used in medicine as they do not stay in the body for a long time, hence

causing minimal damage.

• The rate of radioactive decay is unaffected by any chemical or physical change.

Detection of radioactivity: devices that detect radiation include:

• Cloud chamber: detects alpha and beta particles radiation, leaves a trail of

ions in the water or ethanol vapor (gas) in the chamber.

• Bubble chamber: detects alpha and beta particles radiation, holds a superheated

liquid in which particles leave a path of bubbles if they are present.

• Electroscope: Has leaves that repel or attract each other depending on the

charge in the air. If the electroscope is given a negative charge the metal leaves

separate from each other. A negatively charged electroscope discharges when

ions in the air remove electrons from it, and consequently, a positively charged

electroscope discharges when it takes electrons from the air around it. The

rate of discharge of the electroscope is a measure of ions in the air and can be

used as a basis of measurement and detection.

• Geiger counter: measures radioactivity by producing an electric current when

radiation is present, detects alpha, beta and gamma radiation.

Worked examples

1. If we start with 1.000 grams of Sr-90, 0.953 grams will remain after 2.0

years.

a. What is the half-life of strontium-90?

b. How much strontium-90 will remain after 5.00 years?

15.6. Uses of some radioisotopes

Activity 15.6:

1. According to your own understanding, how do you think radioactivity is

important in daily life?

2. Consult different sources (textbooks and search engines) to carry out a

deep research on the uses of radioisotopes. Please take care because you

will have to present your findings!

Radioactive isotopes have a variety of applications. Generally, they are useful because

either we can detect their radioactivity or we can use the energy they release.

Radioactive isotopes are effective tracers because their radioactivity is easy to detect.

A tracer is a substance that can be used to follow the pathway of that substance

through some structures. For instance, leaks in underground water pipes can be

discovered by running some tritium-containing water through the pipes and then

using a Geiger counter to locate any radioactive tritium subsequently present in the

ground around the pipes.

Tracers can also be used to follow the steps of a complex chemical reaction. After

incorporating radioactive atoms into reactant molecules, scientists can track where

the atoms go by following their radioactivity. One excellent example of this is the

use of carbon-14 to determine the steps involved in photosynthesis in plants.

1. Radioactive Dating

Radioactive isotopes are useful for establishing the ages of various objects. The

half-life of radioactive isotopes is unaffected by any environmental factors, as seen

above, so the isotope acts like an internal clock.

2. In Medicine

Radioactive isotopes have many medical applications in diagnosing and treating

illness and diseases.

When a radiologist wants to determine the condition of an organ in the body, the

patient is given a radioisotope that is known to concentrate in that organ. After

a patient receives a radioisotope, a scanner produces an image of the organ. The

scanner moves slowly across the region of the body where the organ containing the

radioisotope is located. The gamma lays emitted from the radioisotope in the organ

are used to expose a photographic plate with a scan of the organ.

One example of a diagnostic application is using radioactive iodine-131 to test for

thyroid activity. The thyroid gland in the neck is one of the few places in the body

with a high concentration of iodine.

the next day a scanner is used to measure the amount of radioactivity in the thyroid

gland. The amount of radioactive iodine that collects there is directly related to the

activity of the thyroid, allowing radiologists to diagnose both hyperthyroidism and

hypothyroidism.

Iodine-131 has a half-life of only 8 days. So, it has low cell damage due to the

minimum exposure. Technetium-99 can also be used to test thyroid function. Bones,

the heart, the brain, the liver, the lungs, and many other organs can be imaged in

similar ways by using the appropriate radioactive isotope.

Other medical applications of radioisotopes include:

• Radiation from Co-60 (γ-rays) is used to irradiate the tumours (for instance,

diagnosis and treat thyroid disorders).

• Iodine-125 (I-125) is used in treatment of brain cancer and in osteoporosis (a

disease which causes bones to become weaker and easily broken) detection.

• Iodine-131 (I-131) is used to diagnose and treat thyroid disorders, in treatment

of Graves’ disease, goiter and prostate cancer.

• Phosphorus-32 (P-32) is used in the treatment of leukaemia, excess red blood

cells (tumours) and pancreatic cancer.

• Technetium-99m is used in imaging of skeleton and heart muscle, brain, liver,

heart, lungs, bones, spleen, kidney and thyroid. This is the most widely used

radioisotope in nuclear medicine.

• Cerium-141 (Ce-141) is used in gastrointestinal tract diagnosis and in measuring

blood flow to the heart.

• Sodium-24 (Na-24) in the form NaCl is used as a tracer in blood.

• Strontium-85 (Sr-85) is used in detection of bone lesions and brain scans

• Radio Gold (Au-198) is used in Liver disease diagnosis.

• Radio iron (Fe-59) is used in Anemia diagnosis.

• In addition, radioisotopes are also used in sterilization of medical devices.

3. In Agriculture

Obviously, we obtain food to eat and some drinks as a result of agriculture. But

contaminated food causes some diseases. Thus, there are some radioisotopes that

kill dangerous microorganisms present on food by “irradiation”.

The radiation emitted by some radioactive substances can be used to kill

microorganisms on a variety of foodstuffs thereby increasing the shelf life of these

produces. Produces such as tomatoes, mushrooms, sprouts, and berries are irradiated

with the emissions from cobalt-60 or caesium-137. This exposure kills a lot of the

bacteria that could cause spoilage and so the produce stays longer. Eggs and some

meat, such as beef, pork, and poultry, can also be irradiated. Normally, irradiation of

food does not make it radioactive.

By using known vintages (qualities of wines), oenologists (wine scientists) can

construct a detailed analysis of the cesium-137 of various wines through the years.

The verification of a wine’s vintage requires the measurement of the activity of

cesium-137 in the wine. By measuring the current activity of cesium-137 in a sample

of wine (the gamma rays from the radioactive decay pass through glass wine bottles

easily, so there’s no need to open the bottle), comparing it to the known amount of

cesium-137 from the vintage, and taking into account the time passed, researchers

can collect evidence for or against a claimed wine vintage.

In addition in plant research, radiation is used to develop new plant types to speed

up the process of developing large amount of agricultural products. This involves

insect control, drastic reduction of pest populations and, in some cases, elimination

of insects by exposing the male ones to sterilizing doses of radiation. Radiation

pellets are used in grain elevators to kill insects and rodents. Irradiation prolongs the

shelf-life of foods by destroying bacteria, viruses, and molds as seen above.Other

agricultural uses of radioisotopes include the following:

• Radioactive phosphorus (P-32) is used in the study of metabolism of plants.

• Radioactive sulphur (S-35) helps to study advantages and disadvantages of

fungicides.

• Pests and insects on crops can be killed by gamma - radiations.

• Gamma - rays are used for preservation of milk, potatoes etc.

• Yield of crops like carrot, root, apples or grapes can be increased by irradiation

with radioisotopes.

4. In Industry

The applications of radioisotopes in industry are so many. Many types of thickness

gauges exploit the fact that gamma rays are attenuated when they pass through the

material. By measuring the number of gamma rays, the thickness can be determined.

This process is used in common industrial applications such as:

a. The automobile industry: to test steel quality in the manufacture of cars

and to obtain the proper thickness of tin and aluminum

b. The aircraft industry: to check for flaws in jet engines

c. Road construction: to gauge the density of road surfaces and sub surfaces

d. Pipeline companies: to test the strength of welds and leakage

e. Oil, gas, and mining companies: to map the contours of test wells and

mine bores, and

f. Cable manufacturers: to check ski lift cables for cracks.

The isotope 241Am is used in smoke detectors , in thickness gauges designed to

measure and control metal foil thickness during manufacturing processes, to

measure levels of toxic lead in dried paint samples, and to help determine where oil

wells should be drilled.

The isotope 252Cf (a neutron emitter) is used for neutron activation analysis, to

inspect airline luggage for hidden explosives, to gauge the moisture content of soil

and other materials, in bore hole logging in geology, and in human cervix-cancer

therapy.

Checking up 15.6

1. Define tracer and give an example of how tracers work.

2. Explain how radioactive dating works.

3. Name two isotopes that have been used in radioactive dating.

4. The current disintegration rate for carbon-14 is 14.0 Bq. A sample of

burnt wood discovered in an archeological excavation is found to have

a carbon-14 disintegration rate of 3.5 Bq. If the half-life of carbon-14 is

5,730 years, approximately how old is the wood sample?

5. Bone and bony structures contain calcium and phosphorus.

a. Explain why the radioisotopes of calcium-47 and phosphorus-32

would be used in the diagnosis and treatment of bone diseases.

b. During nuclear tests, scientists were concerned that strontium-85,

a radioactive product, would be harmful to the growth of bone in

children. Explain

15.7. Nuclear fission and fusion and their applications

Activity 15.7:

1. Have you ever heard about nuclear fission and fusion? If yes, explain the

two terms and state any use of one of/both these processes

2. From your prior knowledge, explain what you think about the difference

between fission and fusion.

3. Use your search engine, or any available source to read about nuclear

fission and fusion and then make a summary to be presented to your

colleagues.

While many elements undergo radioactive decay naturally, some nuclear reactions

are not spontaneous but are brought about when stable isotopes are bombarded

with high-energy particles (like neutrons, α-particles, protons ...). Nuclear fission

and fusion are good examples artificial radioactivity as they do not take place

spontaneously.

15.7.1. Nuclear fission and fusion

1. NUCLEAR FISSION

Nuclear fission is a process in which a large atomic nucleus is split into two smaller

nuclei. Large nuclei obviously have a large number of protons. The close proximity of

so many protons makes these nuclei unstable due to the repulsion forces between

protons. Thus, the nucleus of the unstable isotope splits to form smaller atoms by

bombardment with a suitable sub-atomic particle. Those stable isotopes that are

bombarded by a neutron to undergo fission reactions (to become fissionable) are

known to be “fertile radioisotopes”.

The neutrons emitted, in this fission reaction, bombard more uranium nuclei

available to form a “reaction chain”. This chain reaction is the basis of nuclear power.

As uranium atoms continue to split, a significant amount of energy is released, in

form of heat, from the reaction. This heat released is used to produce electricity (in a

nuclear plant) or used in atomic/nuclear bombs.

Other examples: nitrogen-14 and oxygen-16 undergoing alpha and neutron

bombardment respectively.

2. NUCLEAR FUSION

Nuclear fusion is a process that consists of joining two atomic nuclei of smaller

masses to form a single nucleus of a larger mass.

A good example is the fusion of two “heavy” isotopes of hydrogen (deuterium,

Hydrogen-2, and tritium, Hydrogen-3) into the element helium.

However, very high temperatures and pressures are required for the fusion to take

place because of the repulsion between the positive nuclei. Thus, for the fusion to

take place the nuclei must have enough kinetic energy to overcome these repulsion

forces between like charges.

Like fission, in the fusion process large quantity of energy is liberated in the form of

heat. This energy is also used in atomic/nuclear bombs (Hydrogen bomb).

Note that it is very difficult to carry out nuclear fusion between two large nuclei due

to the highly strong repulsion forces that are between their positively charged nuclei.

Table 15.6 provides the differences between nuclear fission and nuclear fusion.

15.7.2. Applications of fission and fusion

Both fission and fusion are nuclear reactions that produce energy as described

above. Fission is used in nuclear power reactors since it can be controlled, while

fusion is not utilized to produce power since the reaction has not yet controlled up

to now. The two processes have an important role in the past, present and future in

energy creation.

Example of nuclear energy calculation:

Consider the following nuclear reaction:

• Nuclear fission is non-renewable because uranium or other fissile nuclides

needed for this process are not renewable.

• For example: In Atomic Bomb, there are fission reactions of uranium. The energy

produced is uncontrolled and this principle is used to manufacture the

bombs and missiles. When controlled, the nuclear fission is also useful in the

production of electricity.

• Nuclear fusion energy (if mastered) could be renewable because hydrogen

needed for this process is available in nature in large amount.

• For example: In Hydrogen Bomb, the nuclear reaction involves the fusion of

deuterium and tritium nuclei to form helium.

Checking up 15.7:

1. Classify the following as pertaining to nuclear fission, nuclear fusion, or

both:

a. Small nuclei combine to form larger nuclei.

b. Large amounts of energy are released.

c. Very high temperatures are needed for the reaction.

2. In a fission reaction, U-235 bombarded with a neutron produces Sr-94,

another small nucleus and 3 neutrons. Write the complete equation for

this fission reaction

15.8. Health hazards of radioactive substances

Activity 15.8:

1. Have you ever heard about health risks associated with nuclear radiations?

If yes, state and explain them.

2. Use your search engine or any available resources to you to find out how

we are all exposed to radiation and the hazards that are brought to us by

radiation.

Radioactive materials in the environment, whether natural or artificial, do expose

people to risks.

This can happen in two ways:

• The radiation from the material can damage the cells of the person directly.

This is damage by irradiation.

• Some of the radioactive materials can be swallowed or breathed in. While

inside the body, the radiation it emits can produce damage. This is damage

by contamination.

Some health Hazards are: radiation burns, hair loss (temporary or permanent),

cancer, reproductive sterility, mutations in offspring, etc.

We are all exposed to low levels of radiation every day. Naturally occurring

radioisotopes are part of atoms of wood, brick and concrete in our homes and

the buildings of schools, hospitals, supermarkets, etc. This radioactivity is called

“background radiation” and is present in the soil, in the food we eat, in the water we

drink, and the air we breathe. For instance, one of the naturally occurring isotopes of

present in any potassium-containing food. Other naturally occurring radioisotopes

in air and food are carbon-14, radon-222, strontium-90 and iodine-131.

In addition to naturally occurring radiation from construction materials in our homes,

we are constantly exposed to radiation (cosmic rays) produced in space by the sun.

The larger the dose of radiation received at one time, the greater the effect on the

body. Exposure to low amount of radiation cannot be detected, but at medium

levels the whole-body exposure produces a temporary decrease in number of

white blood cells. If the exposure is very high, the person suffers the symptoms of

radiation sickness such as nausea, vomiting, fatigue, and a reduction of white-cell

count which can even be lowered to zero. So the victim suffers from diarrhea, hair

loss, hair loss and infection. Too much exposure is expected to cause death.

You can ask yourself how radiation (which is in the air all around you) might lead to

lung cancer. This is explained by the presence of Radon gas (mainly from granite rock)

which is the main source of background radiation, and which is in turn responsible

for almost all the radiation we get exposed to over our lifetime. When we breathe in;

some of the radioactive atoms in the air undergo radioactive decay and emit alpha,

beta or gamma radiation. These radiations can collide with and ionize atoms in our

lung tissue, which could ultimately lead to lung cancer.

Figure 15.10: Ways through which radiations reach the body cells

Checking up 15.8:

1. List any three sources of natural radiation.

2. What are some symptoms of radiation sicknesses?

END UNIT ASSESSMENT

I. MULTIPLE CHOICE QUESTIONS. Choose the letter corresponding to the

appropriate answer

1. Elements which emit natural radioactivity are known as:

a. radio elements

b. active elements

c. radioactive elements

d. nuclear elements

2. Spontaneous emission of radiation by unstable nuclei is called

a. positive radioactivity

b. artificial radioactivity

c. natural radioactivity

d. nuclear elements

3. The half-life of technetium-99 is 6 hours. How much of a 100 milligram

sample of technetium-99 will remain after 30 hours?

a. ? 12.5 mg

b. ? 3.125 mg

c. ? 6.25 mg

d. ? 1.56 mg

4. What change occurs in the nucleus of an atom when it undergoes beta

emission?

a. The outer shell of electrons is filled.

b. The number of neutrons decreases by one.

c. A high speed electron is produced.

d. A proton is produced.

e. There is a release of energy.

5. Which one of the following does not occur in nuclear reaction?

a. Nuclear radiation is released.

b. There is a change in mass.

c. It involves a rearrangement of electrons.

d. New elements are made

6 When beta decay occurs in a radioactive isotope, the atomic number (Z)

always

a. increases by one

b. stays the same

c. Increases by two

d. decreases by one

7. Which of the following statements is not correct concerning alphaparticles?

a. they are composed of helium nuclei

b. They are emitted from unstable nuclei

c. they have a positive charge

d. they can penetrate thick sheets of lead

8. Which one of the following is true when a nucleus undergoes radioactive

decay?

a. a new element is always formed

b. alpha-particles are always emitted

c. beta-particles are always emitted

d. the unstable nucleus loses energy

9. Why would the occupants of a house fitted with smoke detectors

containing americium–241 not be at risk from alpha radiation emitted by

these devices?

a. ? It has very low penetrating power through the air.

b. Alpha radiation has very low ionizing power.

c. the occupants wear protective clothing at all times

d. Alpha radiation is not harmful

10. Which of these metals is used as a shield against radioactive emissions?

a. Uranium

b. Lead

c. Radium

d. Gold

b. Half-life

c. Helium

d. Longer

e. Nuclear fission

f. Nuclear fusion

g. Radioactive

h. Radioactive decay

i. Radioactivity

j. Transmutation

1. An element that gives off nuclear radiation is known to be ….

2. When an element changes to another, more stable element, there is …

3. The amount of time for half the atoms in a radioactive sample to decay is

referred to as …

4. The more stable a nucleus is, the ______ its half-life.

5. The process in which the nucleus of an unstable atom releases radiation in

order to become stable is known as …

6. The changing of an atom into another, more stable atom during decay is

…

7. The splitting of an atom into 2 smaller nuclei (nuclear power plant) is

known as …

8. An alpha particle is actually a nucleus of ____________.

9. The name given to the several steps required to get a radioactive element

to a stable element is the …

10. The joining of 2 atoms to form a single, larger nucleus is known as …

III. Short and long answer open questions

1. Define radioactivity.

2. Describe an alpha particle. What nucleus is it equivalent to?

3. Plutonium has an atomic number of 94. Write the nuclear equation for the

alpha particle emission of plutonium-244. What is the daughter isotope?

4. Francium has an atomic number of 87. Write the nuclear equation for the

alpha particle emission of francium-212. What is the daughter isotope?

5. Write balanced equations for the following nuclear reactions:

a. Nuclide carbon-14 undergoes beta decay

b. Uranium-238 decays by alpha particle emission

c. Carbon-11 decays by position emission

d. Cobalt-60 decays by gamma radiation

e. Gold-195 decays by electron capture

6. a) Give values for a, b, c and d in the following nuclear equations:

Calculate the mass number and atomic number of element Y.

7. If radium-226 undergoes a series of decays that produce five α and four β

particles, what is the final product?

8. Strontium-90 is a beta particle emitter and has a half-life of 28.1 yrs.

a. Write the decay equation for strontium-90.

b. Calculate the decay constant

9. A sample of a particular radioactive isotope is separated and monitored

over a period of 15 hours. If it is found that 12.0 grams of the isotope

remain after 4.2 hours and that 10.8 grams remain after 11.3 hours, what is

the half-life of the isotope?

10. What is the age BP of a bone fragment that shows an average of 2.9 dpm/

gC in 2005? The carbon in living organisms undergoes an average of 15.3

dpm/gC, and the half-life of 14C is 5730 years. (BP = Before Present, with

the year 1950 used as the reference; dpm/gC = disintegrations per min.

per gram Carbon)

11. How much energy is released (in kJ) in the following fusion reactions to

yield 1 mol of 4He or 3He?

a. 2H + 3H → 4He + 1n

b. 2H + 2H → 3He + 1n

(The atomic masses are: 1H = 1.00782 u; 2H = 2.01410 u; 3H = 3.01605 u;

3He = 3.01603 u; 4He = 4.00260 u and 1n = 1.008665 u)

12. It has been estimated that 3.9 x 1023 kJ/s is radiated into space by the sun.

What is the rate of the sun’s mass loss in kg/s?

13. How much energy (in kJ) is produced in the fission reaction of 1.0 mol of

uranium-235 according to the following equation?

235U + 1n → 142Ba + 91Kr + 31n

(The atomic masses are: 235U = 235.0439 u; 142Ba = 141.9164 u, 91Kr =

90.9234 u; 1n = 1.00867 u

14. a) State four uses of radioactive isotopes

b) The half-life of cobalt-60 is 5.2 years. What fraction of cobalt-60 would

remain after 26 years?

c) The half-life of carbon-14 is 5600 years. Analysis of a fossil from a historical

site showed that 6.25% of carbon-14 was present compared to living

material. Calculate the age in years of the fossil.

15. The half-life of uranium-238 is 4.5 billion

years. What will be the 238U/206Pb atomic ratio in a rock that is 5.0 billion

years old? (Assume that isotope lead-206 was not present initially)

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