Physical Chemistry Nuclear chemistry B.Sc-I

1. Nuclear Chemistry
B.Sc-I Sem-I
Physical chemistry
Prof. Swapnil S Jadhav
Bhogavati Mahavidyalay Kurukali.

2. Introduction:-
Nuclear Chemistry is sub discipline of
chemistry. It is concerned with changes in
the nucleus of atom. Nuclear changes are
source of radioactivity & nuclear power.
That’s why nuclear chemistry is very
important branch of chemistry.
Atom of the element consists of three
fundamental particles proton, electron and
neutron which are called sub-atomic
particles.

3. These particles are mainly responsible
for physical, chemical and also nuclear
behavior of atoms of all the elements. Out
of them protons and neutrons are jointly
called nucleon.
Nuclear reaction can be brought about
by the interaction of two nuclei or under
the impact of a subatomic particle on the
nucleus. Nuclear chemistry deals with the
study of nuclear particles, nuclear forces
and nuclear reactions.

4. Comparison of Chemical and
Nuclear Reactions:-
Chemical Reactions Nuclear Reactions
One substance is converted into
another, but atoms never
change identity.
Atoms of one element typically
are converted into atoms of
another element.
Only Electrons take part in
chemical reaction
Nucleus of element takes part in
nuclear reaction.
Small amount energy evolved
during chemical reaction
Large amount energy evolved
during nuclear reaction
Reaction rates are influenced by
temperature, concentration and
catalyst.
Reaction rates are depend on
concentration of element, but
not influenced by temperature,
catalyst.

5. Types of Nuclear Radiation:-
Radioactivity:-
Radioactivity is a phenomenon of
spontaneous and uncontrollable disintegration
occurs by emission of active radiations from an
unstable atomic nucleus.
On the basis of effect of electric and
magnetic field the radiations emitted by naturally
occurring radioactive elements are classified into
three types viz., α,  and  –radiations.

6. When these radiations are given out by
radioactive element, the process is called α -
decay,  –decay and  -decay respectively.

7. α –decay:-
"Whenever the element emits an α-particle,
the mass number decreases by 4 units and
atomic number decreases by 2 units."
This is known as α -decay.
α –decay involve an emission of Helium nucleus.

8.  –decay :-
"Whenever the element emits a -
particle, the mass number remains
unchanged and atomic number increases by 1
units."
This is known as  -decay.

9. Actually, the emission of -particle results
due to transformation of a neutron into proton
and electron.
(The electron is ejected from the nucleus while
the proton is retained by it and thus the atomic
number of nucleus increases by one unit)

10.  -decay:-
"Whenever the element emits a -
rays, there is no change either in mass
number or in atomic number."
This is known as  -decay.
 - decay involve radiation of high energy photon.
Metastable Krypton-81 decays by 
emission. This process simply results in lower
energy form of the nucleus.

11. Properties of α ,  and  -rays:-
Properties of α -rays:-
1. These rays consist of +vely charged α -particles.
2. α -particles have +2 charge and mass of 4 units i.e. they
are helium nucleus.
3. They have lower velocity about 1/10th that of the light.
4. They have highest ionizing power due to their
considerable kinetic energy.
5. They possess least penetrating power due to their larger
size and lower velocity.
6. They are deflected in electric and magnetic field to the
smaller extent.
7. They produce luminosity in ZnS due to highest kinetic
energy.
8. They affect photographic plate to lesser extent.

12. Properties of  -rays:-
1. These rays consist of -vely charged  -particles.
2.  -particles have -1 charge and negligible mass i.e. they
are electrons.
3. They have higher velocity about 9/10th that of the light.
4. They have lower ionizing power due to their lower
kinetic energy.
5. They have higher penetrating power due to their small
size and higher velocity.
6. They are highly deflected in electric and magnetic field.
7. They produce lower luminosity in ZnS due to lower
kinetic energy.
8. They affect photographic plate to larger extent.

13. Properties of  -rays:-
1. These rays consist of electromagnetic radiations.
2.  -rays do not carry any charge.
3. They have highest velocity equal to that of the light.
4. They have least ionizing power due to their least
kinetic energy.
5. They have highest penetrating power due to their non-
material nature and higher velocity.
6. They remain undeflected in electric and magnetic
field.
7. They produce least luminosity in ZnS due to negligible
kinetic energy.
8. They affect photographic plate to greater extent.

14. Rate of Radioactive Decay and Decay Constant:-
The number of radioactive atoms disintegrating
per unit time is called rate of radioactive decay or
rate of radioactive disintegration.
It is proportional to the number of radioactive atoms
present at the given instant.

15. Rate Expression (Expression for decay constant):-
Rate of radioactive disintegration varies with
the concentration of radioactive element.
Let, N0 be the number of radioactive atoms present
initially (i.e. when t=0).
Nt be the no. of radioactive atoms present after time t.
If ‘dNt’ number of atoms disintegrates in a given
time of interval ‘dt’, then rate of disintegration
(- dNt /dt’)is given by
ie ……………. (1)
(Negative sign indicates that the number of atoms
decreases with time).

16. λ is proportionality constant and is called
disintegration or decay or radioactive constant.
The decay constant (λ) can be defined as, "the
fraction of the total number of atoms of radioactive
element disintegrating per unit time.”
Rearranging the eqn. (1) we get,
Integrating above equation between the limits, N =N0 at
t=0 & N=Nt at t=t,
we get,

17. Eqn. (2) is the expression for disintegration constant.
(decay constant).

18. The exponential form of Eqn. (2) is
It means that radioactive disintegration is an
exponential process and can be represented
graphically as fallows,

19. According to the exponential
(disintegration) law, infinite time is required for
the complete disintegration of an element.
That is, as time passes, amount of radioactive
element decreases and at infinite time very
negligible amount remains but the amount will
not be reduced to zero.
Radioactive disintegration follows first order
reaction.

20. Half Life and Average Life:-
A) Half life:-
Half life period is defined as, the time required
for disintegration of half of the original amount of the
radioactive substance.
Relation between half-life period and decay
constant:-
Decay constant (λ) is given by the equation,
Where, N0 = initial amount & Nt = amount at time t

21. At half-life period,
t = t1/2 and Nt = N0/2
Hence,
Eqn. (3) is the expression for Half Life.
Since, eq.(3) does not contain any concentration
term; the half-life period is independent of initial
concentration.

22. B) Average Life(T):-
Average or mean life is defined as, the
time up to which the radioactivity of the element
can be appreciably recorded, within experimental
limits.
Alternatively, Average life period (Τ) is nothing
but the reciprocal of decay constant (λ)

23. We have,
Putting this value in above eqn. we get,
Thus,
Average life (Τ) = 1.44 × Half Life period

24. Nuclear Stability, Mass Defect and
Binding Energy, N/Z Ratio:-
Stability of nucleus is affected by the
various factors as fallows.
1. Nuclear forces:-
Nucleus has a very small size (radius 10-10 m)
in which positively charged protons and neutral
neutrons are packed together, but still nucleus is
stable. This is because some strong attractive
forces must be holding these particles together in
the nucleus.

25. There are three types of forces viz.
1. proton-proton (p-p) force,
2. neutron-neutron (n-n) force and
3. proton-neutron (p—n) force.
Collectively, these forces are called nuclear
forces.
(p-p) and (n-n) forces are approximately equal
while (p-n) forces is greater than these two.
Nuclear forces are short-range forces (acting
within the range 10-15 m).

26. Nuclear forces are called exchange forces,
because in the nucleus there is constant
interconversion amongst protons and neutrons
through the formation of mesons (π+, π-, π0)

27. 2. Mass defect and Binding energy:-
A) Mass defect:-
“The difference between calculated mass
and observed atomic mass is called as mass
defect.”
Mathematically it can be calculated by using eqn
Δm = [ZmH + (A-Z) mn] – M
Where,
Δm = mass defect, A = mass number
ZmH = mass Z proton or hydrogen atoms,
(A-Z)mn = mass of (A-Z) neutrons,
M = observed atomic mass.

28. B) Binding energy (B.E.):-
"It is the energy released in binding the
nucleons together in the nucleus."
OR "it is the energy required to break the nucleus
into its isolated nucleons."
This release of energy is due to loss of some mass
and is given by Einstein's equation as,
E = Δmc2
Where,
Δm = mass defect or mass lost C = velocity of light.
If Δm is in grams and C is in cm/sec., then Binding Energy is in
ergs.
If Δm is in kg and C is in m/sec., then Binding Energy is in joules.

29. Generally, Binding Energy is expressed in electron
volts (eV) or million electron volts (MeV).
1 eV = 1.6 x 10-12 ergs = 1.6 x 10-19 joules
1 MeV = 1.6 x 10-6 ergs = 1.6 x 10-13 joules
If the mass expressed in amu, then Binding
Energy (B.E.) in MeV is obtained as,
B.E. = Δm × 931 MeV (1 amu = 931 MeV)
Binding energy per nucleon i.e. average (mean)
binding energy is calculated as,

30. C) Stability and instability of nuclei:-
It is found that nuclei with B.E. between 8 to 9 MeV
are highly stable.
* Nuclei having lower mass and B.E. less than 8 MeV are
unstable and have a tendency for fusion.
* Nuclei having very high mass and B.E. less than 8 MeV are
unstable and have a tendency for fission.

31. From fig. it is clear that,
1. As mass number increases, B.E. increases.
2.The maximum B.E. is 8.7 MeV near mass
number 60 and then it decreases gradually.
Thus, nuclei of elements with very low and
very high mass numbers are unstable.
Nuclei with mass numbers between 20 to 166
and B.E. between 8 to 9 MeV are highly stable.

32. 3. Neutron / Proton (N/Z) Ratio:-
It is observed that neutrons are partly
responsible for the stability of nucleus.
If there are more protons, more neutrons are
required for stability of nucleus.
In plot of N Vs. Z . A line drawn at an angle of 45°
represents nuclei containing N = Z.

33. For light nuclei upto Z= 20, N/Z = 1. For heavy nuclei N/Z > 1.
All stable nuclei have N/Z = 1 to 1.6. This region is called
the zone of stability or stability belt .

34. In a plot of the number of Neutron (N) Vs
atomic number (Z), the stable nuclei fall in a
narrow bond referred as the band of stability.
* Nuclei with N/2>1 are unstable & radioactive
* Elements on LHS of stability zone have N/Z > 1 and
have a tendency to increase protons.
* Elements on RHS of stability zone have N/Z < 1 and
have tendency to decrease protons.

35. Application of Radioisotopes:-
A) As Tracers:-
1. In studying reaction mechanism:-
(i) When water enriched in O18 isotope is used
in photosynthesis, it is found that the oxygen
evolved in the process comes entirely from water
while oxygen of CO2 is retained in organic
compound.

36. (ii) In ester hydrolysis by using water enriched in
O18 isotope, it is found that the acid only contains
excess O18 as,
This indicates that,
-OR' bond is broken. And the -O18H from H2O18
takes the place of —OR’ forming acid while H
combine with —OR’ producing alcohol.

37. 2. In medicine:-
(i) -radiations emitted by 60 Ni, Co60, Co, radium are
useful to prevent the growth of cancer.
(ii) 131I isotope is used to detect and cure the cancers of
thyroid glands.
(iii) 32Pisotope are used to detect cancers (i.e. for
treatment of Leukemia.)
(iv) Radioisotope of iodine is used to detect brain tumor.
(v) 24Na used to determine the efficiency of blood
circulation and function of heart.
(vi) 198Au isotope is used for curing some types of
cancers.
(vii) - radiations are also used to sterilize the surgical
instruments.

38. 3. In agriculture: -
(i) Food grains exposed to -radiations, last longer.
(ii) Superior plant varieties can be obtained by inducing
mutation by  -rays.
(iii) Potatoes and milk are preserved by  -rays.
(iv) Pests and insects on crops can be killed by  - rays.
(v) Radioactive phosphorus is used to study the
efficiency of fertilizers.
(vi) 14C isotope is used for the study of photosynthesis
and biosynthesis.
(vii) 35S isotope is helpful to study the advantages and
disadvantages of fungicides.

39. 4. In industry:-
(i) To measure level of liquids in closed tanks.
(ii) To trace movement of oil in the pipes of a refinery.
(iii) α and  rays are used to measure thickness of
metallic and plastic sheets.
(iv) To study wear and tear of machinery parts.
(v) To study self diffusion of metals, mechanism of
friction and effectiveness of lubricants.
(vi)  -rays are used to detect the flaws and leaks in
moulds, welding and gas systems.

40. B. As radiotherapy:-
(i)  -rays emits by 60Co are used for testing deeply
separated cancer growths.
(ii) Radioisotope of phosphorus is used for treatment of
Leukemia.
(iii) Radioisotope iodine for treatment of
hyperthyroidism.
(iv) 24Na is used to check the blood circulation and to
study the functioning of heart.
(C) In mutation of crops:-
Radioisotopes are used in mutation of crops.
Mutations are induced in plants to get crops with higher
yield, resistant to disease and better adaptability to the
environments.

41. (D) Carbon dating: -
(W. F. Libby (1960) first developed this technique.)
1. The process of determining the age of historic and
archaeological organic samples by comparing the ratio of 14C
to 12C is called 14C dating or carbon dating.
2. The isotope 14C is radioactive. It is produced in upper
atmosphere as,
3. The atmospheric CO2 a mixture of 14CO2 and 12CO2
present in a fixed ratio. Plants absorb this CO2 and prepare
cellulose (wood) by photosynthesis.
4. The ratio 14C to 12C atoms in the living tree is the same
as in the atmosphere.

42. 5. When the tree is cut, this cycle stops and the ratio 14C
to 12C begins to decrease because the 14C atoms are constantly
disintegrating.
6. The concentration of 14C can be measured by counting
its radioactivity.
Consider,
N0 = concentration of 14C in living tree.
Nt = concentration of 14C at particular time t (after cutting).
The age of the wood (or old geological specimen)
(i.e. time, t), can be determined as,
Here, t1/2 Half life period of radioactive carbon (14C).