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Nuclear Changes.

posted Sep 7, 2016, 4:53 PM by Upali Salpadoru   [ updated Mar 16, 2018, 7:52 PM ]

Nuc e.jpg

Fig.1 Nuclear energy to generate electricity.

Matter can undergo three types of changes or reactions, Namely,

 1.Physical changes:-  2. Chemical changes: 3. Nuclear changes:-
  Only the arrangement of molecules change. eg. water to steam.  Making and breaking bonds formed by the electron   sharing or electron exchange. eg. Burning of wood. One kind of matter is changed to another. eg. Hydrogen to Helium.

- Atoms and Isotopes

All matter, living or non living, are made up of only a few more than 100 types of atoms. They are all listed in the Periodic table.

  An Atom

An atom ,basically has 3 kinds of sub atomic particles. They are electrons, protons and neutrons.

 Particle Mass Charge position
 Electron negligible -1 Orbiting far away from nucleus.
 Proton.  1 amu. +1 Inside the nucleus.
 Neutron  1 amu. No Inside the nucleus.

Fig.2 Sub atomic particles in an atom.



Fig.3 Three isotopes of the same element carbon.

An element is identified by the number of protons. Hydrogen 1, Helium 4, Carbon 12 and so on. The chemical properties of an element depend on the number of electrons, which is equal to the number of protons. The neutron number can vary in the same element, The diagram shows 3 types of carbon atoms. As the neutrons have a mass their mass number varies. When using symbols , mass number is at the top while the atomic number is below.

Out of the 3 naturally occurring carbon isotopes only Carbon 14 is radioactive.

Nuclear Changes.

 You must have seen and performed  physical and chemical reactions; but have you ever seen a nuclear reaction? Most probably the answer is NO'
  If so just look at the sun. Sun produces the enormous amount of energy for our solar system by the fusion of hydrogen H to helium He.

There are two kinds of nuclear reactions.

1. Fission:- One element can change into two or more elements.

2. Fusion:-Two elements can fuse together to form a new substance.


This is a term given by Marie Curie for the nuclear reactions that spontaneously take place in nature. But this process was discovered by Henri Becquerel. The rays given out were earlier called Becquerel rays.

In 1896 Henri Becquerel placed a uranium salt above some photographic paper, covered in a  light proof manner and found that the papers behaved as they got exposed to light. On further testing he discovered that the uranium salt was producing some hitherto unknown kind of rays.

Detecting radioactive rays.

Now there are many methods to detect these invisible rays.

1. Photographic paper:- This is what Becquerel discovered.

2. Cloud chamber:

Fig.3 Wilson's cloud chamber.

This is a vessel containing a saturated, or super saturated vapur; usually of alcohol.

The trail of particles or rays become visible due to the condensation of the vapor.

3. Fluorescent screen:-

4. Geiger counter:-

Fig.4.Geiger counter.

When ionising rays enter the tube through the mica window, the gas molecules ionize and allow the current to flow.

Nature of rays and particles produced by Nuclear Reactions.


Fig.5  Deflection of rays by electrical charges.

Uranium  nucleus breaks down emitting powerful rays. They actually consist of three types; Alpha, Beta and Gamma. Their properties are summarized in a table below.

Comparing  α , β  and γ Rays .

Particle / ray.

α Alpha

Helium nuclei

β Beta


γ Gamma



4   amu.


Mass less




No charge


up to

3 x 106 ms-1

up to

27 x 107ms-1

3 x108ms-1

Speed of light


Very high




Very low


Extremely high

Comparing  α , β  and γ Rays .

P p.jpg

Fig.6 Penetrating power of the rays.


Alpha emission.

The nuclear reaction is given here.

Uranium-235 isotope →  Helium nucleus + Thorium-231 isotope.       

In symbols:- ..........  92U235     2α4 + 90Th231

Emitting an alpha particle , (sometimes called an alpha ray), reduces the mass number by 4 and the Atomic number by 2.      

Alpha could not have been the rays that damaged the photo papers of Becquerel, because they have very low penetrating power; unable to go through paper or skin.  

Then what rays exposed the photo papers?

Whenever a nuclear reaction takes place high frequency electromagnetic waves are also produced. These are called Gamma rays γ , and possess a  very high penetrating ability, similar to X rays.

Emitting electromagnetic rays do not change the Mass number or the Atomic number

Moreover  the Thorium produced would have undergone further decay emitting Beta β particles.

Beta emission.

Thorium- 231 →  Beta particle  + Protactinium -231.

In symbols.............. 90Th231 -1β0 + 91Pa231

Emitting a β particle does not change the mass number but it increases the atomic number by 1.

 β rays or particles, are electrons that come from the nucleus. But nucleus does not have electrons. How come they emit e-?
One of the neutrons break up forming a proton and a electron.
 0n1 1p1 + -1e0
                                                          High light to verify your answer. 



This is an amazing substance that glows in the dark emitting various rays for over thousand years. .Marie Sklodowska,(Polish)  and her husband Pierre Curie..Fr., after months of processing tons of Uranium rich  ore, called ,pitchblende, obtained a speck of Radium chloride.  A ton of ore contains only about 0.14 grams of radium.

Fig.7 Ra. was used freely prior to 1960 not knowing the hazards.

Discovering the structure of the atom

Ernest Rutherford used the rays from Radium to get the structure of the atom.
Fig. 5 The plan of the Geiger Marsden experiment that showed the presence of a nucleus inside atoms.
Click Ernest Rutherford year 1907.


When they understood the natural process of changing Uranium to Thorium, Soddy shouted, “This is transmutation, Ernest!” Rutherford said,”Don’t shout Freddy. They’ll call us alchemists

The year was 1902.

Fifteen years later Rutherford was able to perform transmutation by bombarding alpha particles into the nucleus of Nitrogen atoms.

Nuclear Reaction:-

            Nitrogen-14 + Alpha=4 →  Oxygen-17 + Proton-1         In symbols:-                7N14  +2α4 8O17  +1p1
In 1934 Joliot-Curie, son-in-law of Marie Curie, at the Curie Radium Institute in Paris,   obtained Nitrogen -13 and Phosphorous -30 by shooting alpha  to Aluminium.  
“This was the first instance a ‘radioactive isotope’ had been made by man".
2He4 + 13Al37 15P30 + 0n1

Half Life.

This is defined as the time it takes for one-half of the atoms of a radioactive material to disintegrate.

Radioactivity or disintegration of atoms is measured in Becquerels.

One becquerel = 1 radioactive decay per second.


Fig.5.  Disintegration against time graph for the isotope Mendelivium-258.

You should be able to determine the half life of this isotope from the graph.
Answer...50 years. ....  To see the answer high light along the dots



Half life




4.0 ms.




3.7 s

α  and β+

Rn206 and Fr 210

Ra 226

1600 yrs.



Uses of nuclear changes. Using Radio-isotopes.

  1. To generate energy.

The first picture in this lesson shows a plant converting nuclear energy to produce electricity.The first nuclear power plant using the uranium fission began in 1954 in USSR..Today about 17% of the world's electricity is nuclear-generated.

The basic reaction that supply heat to work the turbines is the chain reaction of Uranium-235.


Fig.6.One way of splitting uranium-235.

When a fast emerging neutron from a radioactive material is aimed at the nucleus of a U-235 nuclide, it becomes a u-236 nuclide. This is unstable and undergo fission in different ways. (A nuclide is a nucleus of an isotope)

2. Using Radio-isotopes

  1. As tracer elements-

Sometime it may be necessary to find out where some water goes; it may be in a river, pipe or inside a plant. It is possible to replace one of the H-1 molecules with a H-3 molecule which has a half life of 12 years. A geiger counter can detect these mulecules even after a few years.

Carbon-14, having a half life of  5730 years was used to understand the chemical reactions involved in photosynthesis. This isotope is also used for dating ancient onjects.

  1. Medical applications.

     Fig.7. Marie Curie and her daughter Irène at the Hoogstade Hospital in Belgium, when Radiographic equipment was installed in 1915..  

Courtesey  Nobel            


From the time Pierre Curie damaged his skin by carrying a radium sample in his pocket, Rays from radium had been used to kill cancer cells.


Half life



14 d

Cancer detection and treatment, especially in eyes and skin.


46 d

In studies of iron metabolism in the spleen and  anemia diagnosis.


10.5 mth

Gamma ray irradiation is used to treat cancer.


6 hrs

Imaging the skeleton, heart, brain, thyroid etc,.

To find out the filtration rate in the kidney.


28 days

To label red blood cells.


60 days

For treating thyroid cancer also in diagnosis of abnormal liver function,


15 h

For studies of electrolytes within the body.

C. Home and industry

  1. Gamma sterilisation is used  for food preservation.


Half life



Sr- 90

29 yrs.


In controlling the guage, ( specific thickness) in paper, plastic films, aluminium foil, steel for autos etc.




To detect leaks in pipes.

  1. Smoke alarms The alpha particles pass between the two charged metal plates, causing air particles to ionise (split into positive and negative ions). The ions are attracted to the oppositely charged metal plates causing a current to flow.

Smoke alarm contains metal plates and radioactive substance. The battery supply sends voltage across metal plates. People are alerted when the alarm bell sounds.

. Cross section of a smoke alarm. Positive and negative ions allow a current to flow.

Fig.8.Using Americium-241, an alpha source, to detect smoke.

Courtesy Bitesize  BBC


D. Uses in agriculture.

Fig.9. Phosphorous -32 isotope used to study absorption of fertilizers.

Reference  click  Physics terms.  Then go to R for Radiactivity.

Q.1.0 Identify these:-

Find the most suitable definition for these terms.

2.Nuclear reaction. Nuclear fusion,

  1. Carbon-


Mixed up definitions.


A. Emission of ionizing radiation or particles from a nucleus of a nucleotide.

2. Radioactivity.

C .Forms of the same element that contain equal number of protons but different number of neutrons .

3. Nuclear reaction.

D-Nuclide X226 changing to nuclide Y222B.

4. Nuclear fission.

A- reaction that changes the proton number.

5. Transmutation

E- A reaction that breaks up an atom into two or more atoms.

4x5=20 marks

  1. 0. Study this equation and answer the questions.fission

  1. How many nucleons(Particles inside nucleus)  are there in the reactants.

  2. How many electrons are in the Barium (Ba) atom.

  3. What is the electric charge of ‘n’?

  4. Where should ‘n’ be placed to start the reaction?

  5. Which is the most appropriate term to describe the reaction?

      Select from the following. A- Radiation, B - Fusion. C- Transmutation.

4x5=20 marks

 Q. 3.0Complete these reactions and give a name to describe the type.

  1.        H+ + Cl-  …... .

  2.  Mg24 +   ……. Na24 + H1

  3.   6C14  7N14+.........

  4.  86 Rn222   2H4  +84 Po………

  5. 92U235 + 0n1 + ………

4x5=20 marks


Nr q5.jpg

  1. What do you call different forms of atoms such as U-235 and U-236?

  2. What particle has been added to U-235 to become U-236?

  3. What are the products of splitting U-236?

  4. During stage 4 the sub atomic particles emitted during the splitting of U-236 are made to enter three U-235 atoms. What could be expected to happen in stage 5 and 6?

  5. What word is used to describe a nuclear process such as this?

4x5=20 marks



Half life of a nuclide is  86X210   is   3.7. Answer the following.

  1. What is the number of neutrons in this nuclide.

  2. What probably is the reason for this to be radioactive?

  3. If a sample gave a count rate of 1254, how long would it take to get reduced to a quarter of that count?

  4. When 86X210  disintegrates what would be the nuclide  Y that would result in addition to a helium nucleus?

  5. According to the information supplied what rays or particles may be expected?

        4x5-  =20 marks


  1. 124.

  2. Excess neutrons over a stable ratio.

  3.  3.7 x2 = 7.4 s.

  4.  84Y215

  5.  Alpha and Gamma.

                              For Answers click Answer page Lto K