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Radioactivity - Chemistry Form 4

In this lesson we will discuss Radioactivity.

By the end of the lesson the learner should be able to:
 

-Define radioactivity, half life, radio isotope and Nucleids
-Name the particles emitted during radioactive decay and state their properties
-Write balance nuclear equations

An atom is made up of very small particles called subatomic particles. The main subatomic particles are protons, neutrons and electrons. The protons and neutrons are found in the nucleus of an atom and are referred to as nuclides.

Nuclides may have same number of electrons but different number of neutrons. Such nuclides are called isotopes. Isotopes have the same atomic number but different mass number. An element may have two or more isotopes.

Nitrogen atom

Radioactivity occurs at the atomic level when an atom has too much energy.

Atoms with too much energy release that energy as ''radiation''

Radiation is energy traveling in the form of particles or bundles of energy in a wave form. Some everyday examples of the energy bundles are microwaves used to cook food, radio waves for radio and television, light, and X-rays used in medicine.

The table below shows a summary of some isotopes.



Examples of isotopes

From the table it is possible to have unlimited number of isotopes. Note that, the number of neutrons vary from one isotope to another. However as the number of neutrons become significantly greater than the number of protons the nuclide becomes unstable and may disintegrate spontaneously. For a nuclide to be stable the ratio of neutron to proton should be 1:1 as shown in the table below.


Carbon 14 has 6 protons and 8 neutrons. The ratio of neutrons to protons is therefore 1:1.3. The ratio is significantly greater than 1 therefore C-14 is unstable and likely to disintegrate. The unstable isotopes are reffered to as radioisotopes. For the carbon 14 to gain stability a reaction must occur within the nucleus. This reaction is called a nuclear reaction.

Isotopes of Carbon

Radioactivity is splitting up of an atom to form other nuclide and emitting particles which are accompanied by the release of energy. Radioisotopes are isotopes that undergo radioactivity. Radioactive decay is the spontaneous splitting up of a radio active nuclide as shown in the following animation.


When unstable nuclides disintegrate they form a new nuclei which may have different relative atomic mass and atomic number by emission of particles or energy rays. The spontaneous disintegration of unstable nuclides to form stable with emission of particles or energy rays is called radioactivity.
There are two types of radioactivity

Natural: This is exhibited mainly by isotopes of naturally occurring elements usually with atomic number ranging from 84 to 92 (Polonium to uranium)

Artificial: This exhibited by non radioactive (stable) isotopes after their nucleus are bombarded by subatomic particles (Protons, Neutrons). This is also referred to induced radioactivity.

There are three primary ways in which radioactive isotopes disintegrate (decay) in order to attain stability. These are: alpha (a) emissions, beta (b) emissions and gamma radiation ( l ) emissions.

Alpha particle contains two protons and two neutrons and have a double positive charge. It is identical to a Helium nucleus and is represented as shown;

The following equation shows a alpha emission. When an atom loses an alpha particle the atomic number of the product formed is lower by 2 while the mass number is lower than 4.

The following is an example of an equation showing alpha emission.

Beta particles consists of fast moving electrons. It has a negligible mass and a charge of -1. The mass number remains the same but atomic number is increased by 1. It is identical to an electron. During the decay process a neutron is converted into a proton and an electron. The number of protons in the nucleus reduces and an electron is emitted. It is represented as;

In this reaction Carbon-14 disintegrates into Nitrogen -14 by loose of an electron. The atomic number increases by 1 unit from 6 to 7 in Nitrogen but the mass number remains the same.

The following is an example of an equation showing Beta emission.

This does not contain any particle. It consists of very high energy radiation which are electromagnetic in nature and therefore has no mass or charge.

The table below gives a summary of types of radiation and their charges.

The type of radiation emitted by radioactive isotopes can be identified by its properties such as
Penetrating power
Ionizing ability
Behaviour in a magnetic field
Behavior in an electric field

This refers to how far emission travels through air or in our bodies. Alpha particles are massive and travel slower than the others hence have least penetrating power. They cannot pass through sheets of paper and travel shortest distance in air.

Beta particles has some mass though negligible and travel longer distance in air than alpha particles. They can penetrate through sheets of paper but can be stopped by thin sheet of metal foil. e.g. aluminum foil.
Gamma rays have no mass hence can penetrate through both sheet of paper and metal sheet. They can only be stopped by lead or metal block.

Alpha , beta and Gamma are referred to as ionization radiations because they remove electrons from other elements leaving the nuclei positively charged. Due to the slow speed the rays are capable of ionizing Oxygen and Nitrogen gas more than the other emissions.

Since Alpha and Beta are charged they can be deflected by a magnetic field in opposite direction since they have opposite charges.

In this lesson we will discuss half life.

By the end of the lesson the learner should be able to:

Carry out simple calculation involving half life.

Half life is the amount of time taken for half of the atoms in a sample to decay. The half life for a given isotope is always the same. It does not depend on how many atoms are available. Half Life is denoted as t1\2.For Example: The half life of Berrylium 11 isotope is 13.8 seconds. If its initial amount is 16g, After 13.8 sec half of the initial mass will have decayed leaving a balance of 8gms. In another 13.8 sec half of 8gm will decay leaving a balance of 4gm. This process will continue until all the atoms decay. Click on the Atom below to note how it decays.

The half life of some elements are given below.

Stability of an isotope is determined by its half- life. The shorter the half-life the faster the isotopes decay and the more unstable the isotope is, while the longer the half-life the more stable the isotope. Particles emitted by radioactive isotopes can be counted using the Geiger Muller tube. Half life can be represented graphically as shown below.

The half life of a radioactive Isotope is the time taken for a given mass or number of Nucleids to decay to half its original mass or number. The remaining amount after successive decay can be worked out using the formula.

Remaining Amount = 1/2N x Original Amount

The following is a worked out example showing how to calculate half life.

Click on the ENTER button to follow a worked out example.

The following is a worked out example showing how to calculate half life.

Click on the ENTER button to follow a worked out example.

The following graph shows the decay of Iodine. From the graph given it is possible to determine the half life of Iodine. It is possible to estimate the amount of iodine remaining after a certain number of days. For example after 8 days 50% of the iodine will have remained. After 32 days approximately 5% of the iodine will have remained. Roll over the mouse on each dot to view the amount of Iodine that remains after decay.

The following graph shows the decay of Carbon-14 .From the graph given it is possible to determine the half life of Carbon. It is possible to estimate the amount of Carbon remaining after a certain number of years.

In this lesson we will explain the difference between nuclear fission and fusion, identify uses of radioactivity in day to day life and explain pollution effects of radioactive substances.

By the end of the lesson you should be able to:

i) Distinguish between nuclei fission and fusion
ii) State uses of radioisotopes
iii) State dangers associated with radioactivity

We learnt that nuclear reactions involve emission of small particles namely alpha, beta and gamma rays by radioactive isotopes. However nuclei of some radioisotopes absorb neutrons naturally splitting into two or more stable nuclei.

When uranium 235 nuclide absorbs a neutron, it undergoes a decay process producing two new nuclides, barium -142 and krypton 91 and 3 neutrons. The three (3) neutrons are further absorbed by other uranium 235 nuclei resulting to further split and release of more neutrons which initiates more reactions resulting to a further split and release of more neutrons which initiates more reactions resulting to a CHAIN REACTION and more energy produced.

When a stable nuclide is bombarded with a fast moving neutron, the nuclear splits into two new nuclides with release of energy. The process is called artificial induced radioactivity.
The splitting of heavy unstable nuclei into two or more stable parts after collision with fast moving neutron is referred to as NUCLEAR FISSION while the energy produced is called nuclear energy since it's as a result of nuclear reaction.

The following are examples of reactions involving nuclear fission

The following is an example on nuclear fission.


If the rate of fission is controlled by limiting the neutrons emitted. The energy can be used for power generation from the heat produced.

If the exact products are measured, the product is found to be slightly less than the starting amount.
The lost mass is converted into energy according to Einstein's law E= mc2
Where E= mass defected
C =velocity of light = 3 x 108 ms-1
The nuclear energy produced is so large due to the velocity of light hence a small mass change produces a lot of energy.
The atomic bomb dropped in Hiroshima and Nagasaki in 1945 was as a result of nuclear fission with uranium 235.
If the rate of fission is controlled by limiting the neutrons emitted. The energy can be used for power generation from the heat produced.

We have discussed nuclear fission where nuclides are bombarded with a neutron and broken down into smaller nuclides. Sometimes at high temperature small nuclides with low mass numbers combine to form a heavy nucleus with liberation of large amounts of energy.

Medium nucleus can be formed by fusion of four protons as shown in the equation below.


In nuclear fusion reactions, masses of products are always less than those of reactants same as nuclear fission. The loss of mass is transformed into large amounts of energy. This principal is used in Hydrogen bombs explosions.
Nuclear fusion takes place in the sun and stars. In the sun Hydrogen nuclei fuse at high temperatures to produce Helium and huge amount of Heat and light energy.

Radioactivity has many uses in various fields such as in chemistry, medicine, industry, archeology, agriculture, energy and research.

Determines atoms involved in particular chemical reaction ie reaction mechanisms of chemical reactions.

In medicine radiations are used in the Treatment of cancer and blood disorder.
They are also used in Diagnosis of activities of particular organs e.g. radioactive iodine used to monitor activities of thyroid glands. Technetium 9943TC is used to monitor bone growth. The following animation shows how Gamma rays are used to treat cancer cells.

Sterilizing surgical instrument using gamma rays to kill bacteria.The following video clip shows how surgical instruments are strerilized using Gamma rays. Click to play and observe what happens.(Courtesy of You Tube)




Tracing leaks in gases, water or oil pipelines. The following animation shows how leaks are traced in underground pipes. Click on the play button to view what happens.

Radio active materials are used to check the thickness of paper, rubber and metal sheets. The following animation shows how the Gueller Muller tube is used to check the thickness of paper. Click on the play button to observe what happens.


Checking absorption and use of particular element by plants such as fertilizer uptake. Monitoring growth in plants and synthesis of food products in plants.

Preservation of agriculture products such a perishable food materials, exposure of such food to radiation destroys bacteria, fungi, insects and pests. The following animation shows how the Geiger Muller tube is used to monitor fertilizer uptake in plants. Click the play button.

Carbon 14 is used to determine age of plants and fossils remains and tracing biological processes in plants.

Click to play the following video illustrating how Carbon dating takes place. (Courtesy of You Tube )



Nuclear reactors generate nuclear energy which can be converted to electricity for domestic and Industrial use. This has been used in countries like Korea and Japan.



Nuclear plant

Exposure to high levels of ionizing radiation is harmful to living cells because:
a) Radiation destroys cells and body organs with high energy associated with it if inhaled or swallowed e.g.x-radiation
b) Radiation can ionize and fragment the cells causing genetic mutation, cause cancer of leukemia etc e.g. gamma rays
c) Radiations when allowed to the skin for long period of time cause skin injury since they destroy germination layer where the new skin cells are produced.

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