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CHEMISTRY NEW STUDY MATERIAL-2018-19

UNIT-III

THE NATURE OF NUCLEAR REACTIONS -RADIOACTIVITY

The spontaneous decay of an unstable nucleus with accompanying emission of radiation. Nuclide

- atom with a specific number of protons and neutrons in its nucleus.

There are 271 stable nuclides in nature, others are radioactive radionuclide

- unstable isotope that undergoes nuclear decay.

All isotopes of elements with ≥ 84 protons are radioactive; specific isotopes of lighter.

Elements are also radioactive.

( E.g. H3

1 ) # nucleons = # protons + # neutrons)

Nuclear reactions differ from ordinary chemical reactions

Atomic numbers of nuclei may change (elements are converted to other elements or an element can

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be converted to an isotope of that element).

Protons, neutrons, electrons and other elementary particles may be involved in a nuclear reaction.

Reactions occur between particles in the nucleus.

Matter is converted to energy & huge amounts of energy are released.

Nuclear reactions involve a specific isotope of an element; different isotopes of an element may

undergo different nuclear reactions.

Types of Radioactive Decay:

1) alpha (α) emission :

α particles - high energy and low speed + charged particles; 4 2 He (helium-4)

E.g. emission of an α particle: U238

92 → Th234

90 + He4

2 .

2) beta (β) emission:

β particles ñ high energy and high speed ñ charged electrons: .0

1 e

E.g. emission of a β particle: Th234

90 → Pa234

91 + e0

1 .

• During β decay, a neutron is converted into a proton: n1

0 → P1

1 + e0

1 .

3) Gamma (γ) emission:

Gamma emission accompanies other types of decay γ particles - high energy photons, very

penetrating: Y0

0 .

4) positron ( e0

1 ) emission:

- Same mass, but opposite charge of electron Na22

11 → Ne22

10 + e0

1 .

5) Electron capture - β particle is captured instead of emitted Rb82

37 + e0

1 → Kr82

36.



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Nuclear Stability:

Nuclei containing 2, 8, 20, 50, 82, or 126 protons or neutrons are generally more stable than nuclei

that do not possess these ìmagici numbers.

As the atomic number increases, more neutrons are needed to help bind the nucleus together, so

there is a high neutron: proton ratio.

Nuclei of elements with > 83 protons are unstable due to the large # of nucleons present in the tiny

nucleus; by undergoing radioactive decay unstable nuclei can form more stable nuclei.

Nuclei with both even numbers of both protons & neutrons are generally more stable than those with

odd numbers:

#

protons

#

neutrons

# stable

nuclei

even even 164

even odd 53

odd even 50

odd odd 4

NATURAL RADIOACTIVITY :

Radioactive Decay Series Many heavy elements undergo several sequential emissions before

forming a more stable nuclei:

U238

92 → Th234

90 + He4

2 → Pa234

91 + e0

1 → U238

92 + e0

1 →→→ pb206

82 + He4

2 .

Kinetics of Radioactive Decay

Different isotopes decay at different rates; rates vary from ms to days to years.

Radioactive decay is a first order rate process; all radioactive substances have a characteristic half-

life:

kt1/2 = 0.693 t1/2 = half-life; k = rate constant.



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In ktA

At 0

, A0= initial activity or amount; At = activity after a certain time.

0A

At fraction of material remaining after time t.

NUCLEAR TRANSMUTATION :

Transmutation: -

Change of one element to another as a result of bombardment by highenergy particles.

(E.g. neutrons, electrons, and other nuclei).

Rutherford prepared 1st synthetic nuclide, 17

O, in 1919; Irene Curie prepared 1st radioactive nuclide, 30

P, in 1934.

All trans-Uranium elements (Z > 92) are both synthetic (man-made) and radioactive.

γ-emissions as well as production of protons,Nuclear transmutations can show α−, β−, and neutrons,

and other isotopes:

E.g. Al27

13 + He4

2 → P30

15 + n1

0.

Natural Transmutation Begins with one unstable nucleus that spontaneously decays.

NUCLEAR FISSION :

Fission - A nuclear reaction that releases energy as a result of splitting of large nuclei into smaller

ones. Nuclear Power plants use fission to split U-235 to produce energy:

1. U-235 is bombarded with slow neutrons - this produces smaller nuclei as well as more neutrons

and energy.

2. A chain reaction results because each neutron produced can cause fission of another U235

nucleus.

E.g. U235

92 + n1

0 → Ba142

56 + Kr91

36 + n31

0 .

A nuclear chain reaction is a reaction in which the material that starts the reaction (neutron) is also



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one of the products and can be used to start another reaction.

Nuclear Fusion:

Light mass nuclei combine to form a heavier, more stable nucleus. Nuclear fusion releases even

more energy per gram of fuel then nuclear fission!! (Hydrogen bomb, Sun).

If there are not enough radioactive nuclides in the path of the ejected neutrons, the chain reaction

will die out.

Therefore, there must be a certain minimum amount of fissionable material present for the chain

reaction to be sustained:

Fusion would be a superior method of generating power, if we could make it happen, and if we could

control it here on EARTH.

The products of the reaction are not radioactive, not dangerous!

However, in order to achieve fusion, the material must be in the plasma state at several million

kelvins.

The Nucleus:

The atomic nucleus is made up of protons and neutrons which are collectively called nucleons.

– Nuclide – in nuclear chemistry the atom is referred to as the nuclide.

– Nuclides are represented by two ways: Radium-228 OR 228 (mass number).

The mass defect is caused by the conversion of mass (m) to energy (E) when the nucleus was

originally formed.

Using Einstein’s equation E=mc2 , we can actually calculate the energy that was formed when the

nucleus was formed! This is called the nuclear binding energy.

It can also be thought of as the amount of energy required to break apart the nucleus; therefore, the

nuclear binding energy is also a measure of the stability of a nucleus.

In nuclear equations the total of the atomic number and the total of the mass number must be equal

on both sides of the equation.

Example:

Be9

4 + He4

2 → C12

6 + n1

0.

Mass Number: 9+4 = 13 12+1 = 13

Atomic Number: 4+2 = 6 6+0 = 6

This is also called a transmutation reaction, because the beryllium-9 became carbon-12.



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Fission and Fusion of Atomic Nuclei:

Nuclear Fission:

Fission = The splitting of a nucleus into smaller fragments Uranium-235 and Plutonium-239 are the

only fissionable isotopes.

KEY = In a chain reaction, some of the neutrons produced react with other fissionable atoms,

producing amore neutrons which react with still more fissionable atoms .

Fission of 1 kg of uranium-235 produces the same energy as 20,000 tons of dynamite.

Uncontrolled chain reactions take only fractions of a second to release their energy,

which is why atomic bombs are devices that start uncontrolled chain reactions .

Neutron Moderation Neutron Moderation = process that slows down neutrons so the reactor fuel can

capture.

Them to continue the chain reaction.

Neutron Absorption Neutron Absorption = Process that decreases the number of slow-moving

neutrons which makes the nuclear reaction move slower Nuclear Waste.

KEY = Water cools the spent rods, and also acts as a radiation shield to reduce the radiation levels

Nuclear Fusio.

Fusion = Nuclei combine to produce a nucleus of greater mass.

Atomic bomb :

An atomic bomb is based upon the process of that nuclear fission in which no secondary neutron

escapes the lump of a fissile material for which the size of the fissile material should not be less than a

minimum size called the critical size.

There is accordingly a sudden release of a tremendous amount of energy, which represents an

explosive force much greater than that of the most powerful TNT bomb.

In the world war II in 1945 two atom bombs were used against the Japanese cities of Hiroshima and

Nagasaki, the former contained U-235 and the latter contained Pu-239.

Problem:1

The half-life of polonium-210 is 138.4 days. How many mg of polonium-210 remain after 415. 2

days if you start with 2mg of the isotope?

Solution:

half-lives = 415.2 days X 1 half-life = 3 half-lives 138.4 days



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2mg X ½ X ½ X ½ = 0.25mg.

PROBLEM:2

2. The half-life of radon-222 is 3.824 days. After what time will ¼ of a given amount of radon-222 remain?

Solution:

To obtain ¼ of any amount, it will have to go through 2 half-lives: ½ X ½ = ¼

Critical mass :

minimum mass required to sustain a chain reaction.

Control rods :

control rods are made of B or Cd; these rods absorb neutrons so the process doesn't accelerate too

rapidly. Rods are raised/lowered to control the speed of the process.

Fuel rods :

Fuel rods are made of U-235. 238

U is the most abundant U isotope but is not fissionable so uranium must be

enriched to increase the amount of 235

U.

Moderator :

slows down the neutrons. Water or other liquid coolant surround rods. The water serves to

1) slow down neutrons so they can collide with U-235;

2) transfer heat to steam generator.

Primary problems with nuclear power plants:

1) safety (Chernobyl and Three Mile Island had cooling system failures that led to reactor

meltdowns. Chernobyl also did not have containment building around reactor.)

2) nuclear waste - some products will remain radioactive for thousands of years.

NUCLEAR FUSION:

Fusion :



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A nuclear reaction that releases energy as a result of the union of smaller nuclei to form larger ones.

e.g. H2

1 + H3

1 → He4

2 + n1

0 .

Fusion generates even more energy than fission and creates little radioactive waste, so it would

provide a wonderful source of energy.

Fusion requires very high temps (tens of millions of degrees Celsius) in order for nuclei to overcome

strong repulsive forces ñ magnetic fusion reactors are being designed and tested.

APPLICATIONS OF RADIOACTIVE ISOTOPES:

Nuclear power plants

Medical diagnosis and treatment

E.g. PET scan monitors glucose metabolism in brain using C-11 isotope; I-131 measures activity of

thyroid.

Carbon dating (measure amount of C-14 remaining in a sample)

Synthesis of new elements

Irradiation of food - preserves food & destroys parasites

Nuclear Weapons (Atomic bombs and H bombs).

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TRB-POLYTECHNIC- LECTURER

CHEMISTRY MODEL TEST

TIME DURATION:1/2 HOUR MAX.MARKS:30

_______________________________________________________________________________________ 1.Radio-activity was discovered by

a) J.J. Thomson

b) Madame Curie

c) Henry Becquerel

d) Rutherford

2.When a radioactive substance is subjected to a vacuum, the rate of disintegration per second

a) increases only if the products are gaseous

b) increase considerably

c) decreases

d) is not affected

3.Gamma rays are

a) Low energy waves



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b) High energy protons

c) High energy electron

d) High energy electromagnetic waves

4.Who observed that when the nucleus of uranium atom was bombarded with fast moving neutrons, it

becomes so very unstable that it is immediately broken into two nuclei of nearly equal mass besides

other fragments?

a) Chadwick

b) Einstein

c) Rutherford

d) J.J. Thomson

5.Out of the following the one which has no charge is

a) Gamma rays

b) Beta rays

c) Alpha rays

d) Cathode rays

6.Hydrogen bomb is based on the phenomenon of

a) Nuclear explosion

b) Chemical reaction

c) Nuclear fusion

d) Nuclear fission

7.When a radioactive nucleus emits an alpha particle, the mass number of the atom

a) remains same and its atomic number decreases

b) decreases and its atomic number increases

c) decreases and its atomic number decreases

d) increases and its atomic number decreases

8.Artificial radioactivity was discovered by

a) Hahn and Strassman

b) Joliot and Irene Curie

c) Eienstein

d) Rutherford



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9.Uranium gives stable isotope of

a) radon

b) krypton

c) polonium

d) radium

10.Positron emission results from the transformation of one nuclear proton into a neutron. The

isotope thus produced possesses

a) intense radioactivity

b) higher nuclear charge

c) no radioactivity

d) same mass number

11.The half-life of a radioactive element is 50 days. How long will it take far its activity reduce to 1/4

of it original value?

a) 100 days

b) 75 days

c) 50 days

d) 25 days

12.The age of most ancient geological formation is estimated by

a) Uranium lead method

b) Radium-silicon method

c) Carbon 14 dating method

d) Potassium-Argon method

13.What is the fuel of atomic pile?

a) Uranium

b) Petroleum

c) Sodium

d) Thorium

14.If half life period of radium is 1580 years, its average life period will be

a) 1275 years

b) 2175 years

c) 2275 years



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d) 2400 years

15.Who gave the theory of radioactive disintegration?

a) Thomson and Einstein

b) Hahn and Strassmann

c) Soddy and Fajan

d) Rutherford and Soddy

16.The unit of radioactivity is

a) Becquerel

b) Rutherford

c) Curie

d) both Curie and Rutherford

17.Atom A possesses higher values of packing fraction than atom B. the relative stabilities of A and B

are

a) B is more stable than A

b) A is more stable than B

c) A and B both are equally stable

d) Stability does not depend on packing fraction

18.Radium and chlorine combine to form radium chloride, the compound is

a) as radioactive as the radium content

b) half as radioactive as the radium content

c) thrice as radioactive as the radium content

d) no longer radioactive

19.The nucleus of radioactive element possesses

a) zero binding energy

b) high potential energy

c) high binding energy

d) low binding energy

20.After two hours one sixteenth of the original quantity of a certain radioactive substances remains

unchanged. The half life period of the substance is

a) 30 minutes



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b) 40 minutes

c) 25 minutes

d) 45 minutes

21.Atom bomb is based on the principle of

a) Nuclear fission

b) Nuclear fusion

c) Fusion and fission both

d) Radioactivity

22.Nuclear energy is based on the conversion of

a) uranium into radium

b) neutrons into protons

c) mass into energy

d) protons into neutrons

23.When the quantity of a radioactive substance is increased two times, the number of atoms

disintegrating per unit time is

a) increased by square of two

b) increased, but not a great extent

c) doubled

d) tripled

24.Penetrating power of proton is

a) greater than neutron

b) lesser than electron

c) greater than electron

d) None

25.Large energy released in an atomic bomb explosion is mainly due to

a) products having a lesser mass then initial substance

b) release of electrons

c) release of protons

d) conversion of heavier to lighter atoms

26.The average life period of a radioactive element is



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a) 1.44 T

b) 144 T

c) 0.144 T

d) 14.4 T

27.Emission is caused by the transformation of one neutron into a proton. This results in the

formation of a new element having

a) lower nuclear charge

b) same nuclear charge

c) nuclear charge lower by one unit

d) nuclear charge higher by one unit

28.A certain nucleide has a half life period of 30 minutes. If a sample containing 600 atoms is allowed

to decay for 90 minutes, how many atoms will remains.

a) 175 atoms

b) 250 atoms

c) 450 atoms

d) 75 atoms

29.A radioactive isotope has a half life of 20 days. If 100 gms. of the substance is taken the weight of

the isotope remaining after 40 days is

a) 2.5 gms

b) 15 gms

c) 25 gms

d) 75 gms

30.Nuclides having the same number of protons and same number of neutrons but different in

radioactivity are called

a) Isotones

b) Isobars

c) Nuclear isomers

d) Isodiapheres.



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1. c 2. d 3. d 4. a 5. a

6. c 7. c 8. b 9. c 10. d

11. a 12. a 13. a 14. c 15. d

16. d 17. a 18. a 19. d 20. a

21. a 22. c 23. c 24. c 25. a

26. a 27. d 28. d 29. c 30. c

31. b 32. d 33. a 34. a 35. b

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