Nuclear chemistry

Physical Chemistry B.Sc-I

1. Nuclear Chemistry Prof. Jadhav S.S. MSc.BEd.


Introduction:-
Nuclear Chemistry is sub discipline of chemistry. i.e. 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. 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.

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.

2. 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.





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

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.
7
=6
67<
6ℎ
=4
678
+ *A
6
8


2Q
=8
67=
7
=6
679
+ *A
6
8


α –decay involve emission of Helium nucleus.

3.  –decay :-
"Whenever the element emits a -particle, the mass number remains
unchanged and atomic number increases by 1 units."
This is known as  -decay.
J
4
5
L
5
5
+ A
?5
4
(
?5
4
)

*
5
7
*A
6
7
+ A
?5
4


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)

+
97
575
:A
98
575
+ A
?5
4


 -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.

-N
7:
<5
-N
7:
<5
+ 
4
4

4. Properties of α ,  and  -rays:-

Properties of α -rays:-
1. These rays consist of +vely charged particles called 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 1/10
th
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 produce luminosity in ZnS due to highest kinetic energy.
7. They are deflected in electric and magnetic field to the smaller extent.
8. They affect photographic plate to lesser extent.

Properties of  -rays:-
1. These rays consist of -vely charged particles called  -particles.
2.  -particles have -1 charge and negligible mass i.e. they are electrons.
3. They have higher velocity about 9/10
th
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 produce lower luminosity in ZnS due to lower kinetic energy.
7. They are highly deflected in electric and magnetic field.
8. They affect photographic plate to larger extent.

Properties of  -rays:-
1. These rays consist of electromagnetic radiations like -rays.
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 produce least luminosity in ZnS due to negligible kinetic energy.
7. They remain undeflected in electric and magnetic field.
8. They affect photographic plate to greater extent.

5. 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.

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 number of radioactive atoms present after time t.
If ‘dNt’ number of atoms disintegrates in a given time of interval ‘dt’, then
rate of disintegration
????
??
is given by


????
??
∝ Nt
(Negative sign indicates that the number of atoms decreases with time).


????
??
= λ Nt ……………………….. (1)

λ 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 eq
n
. (1) we get,

???
??
= - λ dt
Integrating above equation between the limits, N =N0 at t=0 & N=Nt at t=t, we
get,
∫
???
??
??
?4
= - λ ∫@P

loge
??
?4
= - λ t ( ∫
5
?
@T = loge T )

6. i.e. λ t = - loge
??
?4


. λ t = loge
?4
??

.
λ =
5
?
loge
?4
??


Thus, λ =
6.747
?
log
?4
??
……………………………… (2)

Eq
n
. (2) is the expression for disintegration constant (decay constant).

The exponential form of Eq
n
. (2) is Nt = N0 A
??r


It means that radioactive disintegration is an exponential process and can be
represented graphically as fallows,



Fig.:- Disintegration of radioactive element.

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, at infinite time very negligible amount remains but
the amount will not be reduced to zero.
Radioactive disintegration follows first order reaction.

7. 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,
λ =
6.747
?
log
?4
??


Where, N0 = initial amount and Nt = amount at time t
At half-life period,
t = P
5/6 and Nt = N0/2
Hence,
λ =
6.747
?-/.
log
?4
R4/6


λ =
6.747
?-/.
log2

λ =
6.747 × 4.7454
?-/.


λ =
4.:=7
?-/.


?
?/? =
?.???
?
……………………………….. (3)

Eq
n
. (3) is the expression for Half Life.
Since, equation (3) does not contain any concentration term; the half-life period is
independent of initial concentration.

8. B) Average Life:-
Average or mean life is defined as, the time up to which the radioactivity
of the element can be appreciably recorded, within experimental limits.

Average life =
?? ?? ????? ?? ?? ?????
????? ?????? ?? ?????


Alternatively,
Average life period (Τ) is nothing but the reciprocal of decay constant (λ)

Average life (Τ) =
5
?


We have, P
5/6 =
4.:=7
?


i.e. λ =
4.:=7
?-/.


Putting this value in above eq
n
. we get,

Average life (Τ) =
5
?
=
?-/.
4.:=7
= 1.44 × P
5/6 ……………………. (4)
Thus
Average life (Τ) = 1.44 × Half Life period

9. 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 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.
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).
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
-
, π
0
)



n

P n P
Due to these exchange forces, the nucleons adjust themselves to form stable
nucleus of minimum potential energy.

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 eq
n


Δm = [ZmH + (A-Z)mn] – M
Where,
Δm = mass defect, ZmH = mass Z proton or hydrogen atoms, A = mass number
(A-Z)mn = mass of (A-Z) neutrons, M = observed atomic mass.

+ π
+
- π
+

10. 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 = Δmc
2

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.
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 masses are expressed in amu, then Binding Energy (B.E.) in MeV is
obtained directly by using the relation,
B.E. = Δm × 931 MeV (since, 1 amu = 931 MeV)

binding energy per nucleon i.e. average (mean) binding energy is calculated as,

B.E. per Nucleon =
?? × =75
?
MeV

11. 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.



From fig. It is clear that, as mass number increases, B.E. increases. The
maximum B.E. is 8.7 MeV near mass number about 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 and 166 as well as B.E. between 8
to 9 MeV are highly stable.

12. 3. Neutron / Proton (N/Z) Ratio:-
It is observed that neutrons are partly responsible for the stability of
nucleus because except hydrogen isotope no nucleus contains only protons. If there
are more protons in the nucleus more neutrons are required per proton for stability
of nucleus.
This can be seen from a plot of N Vs. Z . A line drawn at an angle of
45° represents nuclei containing equal number of N and Z.



For light nuclei upto Z = 20, the ratio N/Z = 1. For heavy nuclei this ratio is larger
than unity and increases with increase in Z because the number of neutrons
exceeds the number of protons.
All stable nuclei lie within a certain range of N/Z ratio of about 1 to 1.6. This
region is called the zone of stability or stability belt
In a plot of the number 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 R.H.S.of stability zone have N/Z < 1 and have tendency to decrease
protons.
?
?
> 1
?
?
< 1

13. Application of Radioisotopes:-
A) As Tracers:-
1. In studying reaction mechanism:-
(i) When water enriched in O
18
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. Application of Radioisotopes:-

CO
6
5:
+

H2O
18

5
:

(C6H12O
:
5:
) + O
6
5<

(ii) In ester hydrolysis by using water enriched in O
18
isotope, it is found that the
acid only contains excess O
18
as,
R--CO—OR
’
+ HO
18
H R--CO—O
18
H + R’—OH
This indicates that, -OR' bond is broken and O
18
H from H2 O
18
takes the place of —OR
’

while H combine with —OR
’
producing alcohol.

2. In medicine:-
(i) Strong -radiations emitted by
60
Ni, Co
60
, Co, radium etc. are useful to prevent the
growth of cancer.
(ii)
131
I isotope is used to detect and to cure the disorders or cancers of thyroid glands.
(iii)
32
P

isotope are used to detect cancers i.e. for treatment of Leukemia.
(iv) Radioisotope of iodine is used to detect brain tumor,
(v)
24
Na to determine the efficiency of blood circulation as well as function of heart.
(vi)
198
Au isotope is used for curing some types of cancers.
(vii) - radiations are also used to sterilize the surgical instruments.

14. 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  - radiations.
(v) Radioactive phosphorus is used to study the efficiency of fertilizers.
(vi)
14
C isotope is used for the study of photosynthesis and biosynthesis.
(vii)
35
S isotope is helpful to study the advantages and disadvantages of fungicides.

4. In industry:-
(i) To study self diffusion of metals, mechanism of friction and effectiveness of
lubricants.
(ii)  -rays from radioisotopes are used to detect the flaws and leaks in moulds, welding
and gas systems.
(iii) To measure level of liquids in closed tanks and to trace movement of oil in the
pipes of a refinery.
(iv) α and  rays are used to measure thickness of metallic and plastic sheets.
(v) To study wear and tear of machinery parts by using radioactive tracers.
B. As radiotherapy:-
(i)
60
Co emits  -rays 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)
24
Na is used to check the blood circulation and to study the functioning of
heart.

15. (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.

(D) Carbon dating: - (W. F. Libby (1960) first developed this technique.)
The process of determining the age of historic and archaeological organic
samples by comparing the ratio of
14
C to
12
C is called
14
C dating or carbon
dating.
The isotope
14
C is radioactive. The
14
C atom is produced in upper atmosphere
by the bombardment of neutron on Nitrogen atom.
N
;
58
+ n
4
5
C
:
58
+ H
5
5


The atmospheric carbon dioxide a mixture of
14
CO2 and
12
CO2 present in a
fixed ratio. Plants absorb CO2 from the atmosphere and prepare cellulose
(wood) by photosynthesis. As long as the plant is alive the ratio of
14
C to
12
C
atoms in the wood is the same as in the atmosphere.
When the tree is cut, this cycle stops and the ratio
14
C to
12
C begins to
decrease because the
14
C atoms are constantly disintegrating. The concentration of
14
C can be measured by counting its radioactivity.
Consider, N0 concentration of
14
C in fresh (living) tree
Nt concentration of
14
C at particular time t (after cutting),
The age of the wood or old geological specimen (i.e. time, t), can be determined
by using disintegration law, as
λ =
6.747
?
log
?4
??
where, λ = 0.693/ P
5/6
here, P
5/6 Half life period of radioactive carbon (
14
C)