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“The branch of chemistry which deals with the
study of composition of atomic nucleus and the nuclear
transformations is known as nuclear chemistry”.
The common examples of nuclear processes are
radioactivity, artificial transmutations, nuclear fission
and nuclear fusion. The nuclear is also an important
aspect of chemistry because the energies involved in
some of these are million times greater than those in
ordinary chemical reactions.
Radioactivity
“Radioactivity is a process in which nuclei of
certain elements undergo spontaneous disintegration
without excitation by any external means .’’ and the
elements whose atoms disintegrate and emit radiations
are called radioactive elements.
Henry Becquerel (1891) observed the spontaneous
emission of invisible, penetrating rays from potassium
uranyl sulphate K 2 UO 2 (SO 4 ) 2 , which influenced
photographic plate in dark and were able to produce
luminosity in substances like ZnS.
Later on, M.M. Curie and her husband P. Curie
named this phenomenon of spontaneous emission of
penetrating rays as, Radioactivity.
Curies also discovered a new radioactive element
Radium from pitchblende (an ore of U i.e. U 3 O 8 ) which
is about 3 million times more radioactive than uranium.
Now a days about 42 radioactive elements are known.
The radioactivity may be broadly classified into
two types,
(1) If a substance emits radiations by itself, it is
said to possess natural radioactivity.
(2) If a substance starts emitting radiations on
exposure to rays from some natural radioactive
substance, the phenomenon is called induced or
artificial radioactivity.
Radioactivity can be detected and measured by a
number of devices like ionisation chamber, Geiger
Muller counter, proportional counter, flow counter, end
window counter, scintillation counter, Wilson cloud
chamber, electroscope, etc.
Nature and characteristics of radioactive
emissions
The phenomenon of
radioactivity arises because
of the decay of unstable
nuclei or certain element.
The nature of the radiations
emitted from a radioactive
substance was investigated
by Rutherford (1904) by
applying electric and
magnetic fields. When these
radiation were subjected to electric or magnetic field,
these were split into three types , and – rays.
Characteristics of radioactive rays
- Ray - Ray - Ray
Charge and mass : It
carries +2 charge and 4 unit mass.
It carries - 1
charge and no mass.
It has no
charge and negligible
mass.
Identity : Helium
nuclei or helium
ion
4 2 He or He 2+ .
Electron
0 ^1 e High energy raditons.
Action of magnetic field : Deflected
towards the cathode.
Deflected to anode.
Not deflected.
Velocity : 1/ th to
that of light.
Same as that of
light.
Same as that
of light.
Nuclear Chemistry
Chapter
Radioactive Lead block substance
Photographic plate
Fig. 7.
Ionizing power : Very high nearly 100 times to that of -rays.
Low nearly 100 times to that of - rays.
Very low.
Effect on ZnS plate : They cause
luminescence.
Very little effect. Very little effect.
Penetrating power : Low
100 times that of -particles.
10 times that of -particles.
Range : Very small. More than -
particles.
More
Nature of product :
Product obtained by the loss of 1 -particle
has atomic number less by 2 units and mass number less by 4
units.
Product obtained
by the loss of 1 - particle has
atomic number more by 1 unit, without any
change in mass number.
There is no
change in the atomic
number as well as in mass
number.
Theory of radioactivity disintegration
Rutherford and Soddy , in 1903, postulated that
radioactivity is a nuclear phenomenon and all the
radioactive changes are taking place in the nucleus of
the atom. They presented an interpretation of the
radioactive processes and the origin of radiations in the
form of a theory known as theory of radioactive
disintegration. The main points of this theory are,
(1) The atomic nuclei of the radioactive elements
are unstable and liable to disintegrate any moment.
(2) The disintegration is spontaneous, i.e. ,
constantly breaking. The rate of breaking is not
affected by external factors like temperature, pressure,
chemical combination etc.
(3) During disintegration, atoms of new elements
called daughter elements having different physical and
chemical properties than the parent elements come into
existence.
(4) During disintegration, either alpha or beta
particles are emitted from the nucleus.
The disintegration process may proceed in one of
the following two ways,
(i) - particle emission : When an -particle
( )
4 2 He^ is emitted from the nucleus of an atom of the
parent element, the nucleus of the new element, called
daughter element possesses atomic mass or atomic
mass number less by four units and nuclear charge or
atomic number less by 2 units because -particle has
mass of 4 units and nuclear charge of two units.
2
4 :
:
Z
W Atomic numberZ
Atomic massW
Parent element Daughterelement
- α
(ii) - particle emission : -particle is merely an
electron which has negligible mass. Whenever a beta
particle is emitted from the nucleus of a radioactive
atom, the nucleus of the new element formed possesses
the same atomic mass but nuclear charge or atomic
number is increased by 1 unit than the parent element.
Beta particle emission is due to the result of decay of
neutron into proton and electron.
0 1
1 1
1 0 n^ p e
The electron produced escapes as a beta-particle-
leaving proton in the nucleus.
: 1
:
Z
W Atomic numberZ
Atomic massW
Parent element Daughterelement
- β
(iii) - ray emission : -rays are emitted due to
secondary effects. The excess of energy is released in
the form of -rays. Thus - rays arise from energy re-
arrangements in the nucleus. As -rays are short
wavelength electromagnetic radiations with no charge
and no mass, their emission from a radioactive element
does not produce new element.
Special case : If in a radioactive transformation 1
alpha and 2 beta-particles are emitted, the resulting
nucleus possesses the same atomic number but atomic
mass is less by 4 units. A radioactive transformation of
this type always produces an isotope of the parent
element.
4 4 1
4 2
W Z
W Z
W Z
W Z A^ B C D
α β β
A and D are isotopes.
Group displacement law
Soddy, Fajans and Russell (1911-1913) observed
that when an -particle is lost, a new element with
atomic number less by 2 and mass number less by 4 is
formed. Similarly, when -particle is lost, new element
with atomic number greater by 1 is obtained. The
element emitting then or -particle is called parent
element and the new element formed is called daughter
element. The above results have been summarized as,
(1) When an -particle is emitted, the new
element formed is displaced two positions to the left in
the periodic table than that of the parent element
( because the atomic number decreases by 2 ).
(2) When a -particle is emitted, the new element
formed is displaced one position to the right in the
periodic table than that of the parent element ( because
atomic number increased by 1 ).
(3) When a positron is emitted, the daughter
element occupies its position one group to the left of
the parent element in periodic table.
Group displacement law should be applied with
great care especially in the case of elements of
lanthanide series (57 to 71), actinide series (89 to 103),
VIII group (26 to 28; 44 to 46; 76 to 78), IA and IIA
groups.
To determine the number of - and - particles
emitted during the nuclear transformation. It can be
done in following manner,
0 1
4 X Y x 2 He y e
b d
a c
a b 4 x or 4
a b x
.....(i)
value of binding energy is negative, the product nucleus
or nuclei will be less stable than the reactant nucleus.
Thus the relative stability of the different isotopes of
an element can be predicted by the values of binding
energy for each successive addition of one neutron to
the nucleus.
He n He 20. 5 MeV 4 2
1 0
3 2
He n He 0. 8 MeV
5 2
1 0
4 2
Therefore,
4 2 He is more stable than
3 2 He and
5 2 He.
(2) Packing fraction : The difference of actual
isotopic mass and the mass number in terms of packing
fraction is defined as,
4 10 Massnumber
Actualisotopicmass Massnumber Packing fraction
The value of packing fraction depends upon the
manner of packing of the nucleons with in the nucleus.
Its value can be negative, positive or even zero. A
negative packing fraction generally indicates stability
of the nucleus.
In general, lower the packing fraction, greater is
the binding energy per nucleon and hence greater is the
stability. The relatively low packing fraction of He, C
and O implies their exceptional stability, packing
fraction is least for Fe (negative) and highest for H
( +78 ).
(3) Magic number : Nucleus of atom, like extra-
nuclear electrons, also has definite energy levels
(shells).
Nuclei with 2, 8, 20, 28, 50, 82 or 126 protons or
neutrons have been found to be particularly stable with
a large number of isotopes. These numbers, commonly
known as Magic numbers are defined as the number of
nucleons required for completion of the energy levels
of the nucleus. Nucleons are arranged in shells as two
protons or two neutrons (with paired spins) just like
electrons arranged in the extra-nuclear part. Thus the
following nuclei
40 20
16 8
4 2 He , O , Ca and
208 82 Pb
containing protons 2, 8, 20 and 82 respectively (all
magic numbers) and neutrons 2 , 8, 20 and 126
respectively (all magic numbers) are the most stable.
Magic numbers for protons : 2, 8, 20, 28, 50,
82,
Magic numbers for neutrons : 2, 8, 20, 28, 50, 126,
184, 196
When both the number of protons and number of
neutrons are magic numbers, the nucleus is very stable.
That is why most of the radioactive disintegration
series terminate into stable isotope of lead (magic
number for proton = 82, magic number for neutron =
126). Nuclei with nucleons just above the magic
numbers are less stable and hence these may emit some
particles to attain magic numbers.
(4) Neutron-proton ratio or causes of
radioactivity It has been found that the stability of
nucleus depends upon the neutron to proton ratio
( n/p ). If we plot the number of neutrons against
number of protons for nuclei of various elements, it has
been observed that most of the stable (non-radioactive)
nuclei lie in a belt shown by shaded region in figure
this is called stability belt or stability zone. The nuclei
whose n / p ratio does not lie in the belt are unstable and
undergo spontaneous radioactive disintegration.
It has been observed that,
(i) n / p ratio for stable nuclei lies quite close to
unity for elements with low atomic numbers (20 or
less) but it is more than one for nuclei having higher
atomic numbers. Nuclei having n / p ratio either very
high or low undergo nuclear transformation.
(ii) When n / p ratio is higher than required for
stability, the nuclei have the tendency to emit rays
i.e., a neutron is converted into a proton.
(iii) When n / p ratio is lower than required for
stability, the nuclei increase the ratio, either by
emitting particle or by emitting a position or by K -
electron capture.
Rate of radioactive decay
“ According to the law of radioactive decay, the
quantity of a radio-element which disappears in unit
time ( rate of disintegration ) is directly proportional to
the amount present .”
The law of radioactive decay may also be
expressed mathematically.
Suppose N 0 be the number of atoms of the
radioactive element present at the commencement of
observation, t 0 and after time t , the number of atoms
remaining unchanged is Nt
. The rate of disintegration
dt
dN t at any time t is directly proportional to N.
Then, dt
dN (^) t = N
n/p=
Proton rich nuclei
Stability belt
Neutron rich nuclei
Neutron number (
n
)
Atomic number
( p )
20 40 60 80100 120
Fig. 7.
where is a radioactive constant or decay
constant.
Various forms of equation for radioactive decay
are,
t Nt Ne
0 ;log N (^) 0 log Nt 0. 4343 t
log
0 t
N
N
t
Nt
N
t
0 log
This equation is similar to that of first order
reaction, hence we can say that radioactive
disintegration are examples of first order reactions.
However, unlike first order rate constant ( K ), the decay
constant ( ) is independent of temperature.
Rate of decay of nuclide is independent of
temperature, so its energy of activation is zero.
(1) Half-life period ( T 1/2 or t 1/2) : The half-life
period of a radioelement is defined, as the time
required by a given amount of the element to decay to
one-half of its initial value.
- 693 t 1 / 2
Now since is a constant, we can conclude that
half-life period of a particular radioelement is
independent of the amount of the radioelement. In
other words, whatever might be the amount of the
radioactive element present at a time, it will always
decompose to its half at the end of one half-life period.
Let the initial amount of a radioactive substance
be N 0
Amount of radioactive substance left after n half-
life periods
0 2
N N
n
Total time T n t 1 / 2 where n is a whole number.
(2) Average-life period ( T ) : Since total decay
period of any element is infinity, it is meaningless to
use the term total decay period (total life period) for
radioelements. Thus the term average life is used.
Average life ( T ) Totalnumberofnuclei
Sumoflivesof thenuclei
Average life ( T ) of an element is the inverse of its
decay constant, i.e.,
1 T , Substituting the value of
in the above equation,
1 / 2
1 / 2
- 44
- 693
t
t T
Thus, Average life ( T )
1. 44 Halflife( T 1 (^) / 2 ) 2 t 1 / 2
Thus, the average life period of a radioisotope is
approximately under-root two times of its half life
period.
(3) Activity of population or specific activity : It
is the measure of radioactivity of a radioactive
substance. It is defined as ' the number of radioactive
nuclei, which decay per second per gram of radioactive
isotope.' Mathematically, if ' m ' is the mass of
radioactive isotope, then
m g
N
m Atomicmassin
Rateofdecay Avogadronumber Specific activity
where N is the number of radioactive nuclei
which undergoes disintegration.
(4) Radioactive equilibrium : Suppose a
radioactive element A disintegrates to form another
radioactive element B which in turn disintegrates to
still another element C.
A B C
B is said to be in radioactive equilibrium with A if
its rate of formation from A is equal to its rate of decay
into C.
It is important to note that the term equilibrium
is used for reversible reactions but the radioactive
reactions are irreversible, hence it is preferred to say
that B is in a steady state rather than in equilibrium
state.
At a steady state,
B
A
B
A
A
B
B
A
t
t
T
T
N
N
1 / 2
1 / 2
Thus at a steady state (at radioactive equilibrium),
the amounts (number of atoms) of the different
radioelements present in the reaction series are inversely
proportional to their radioactive constants or directly
proportional to their half-life and also average life
periods.
(5) Units of radioactivity : The standard unit in
radioactivity is curie ( c ) which is defined as that
amount of any radioactive material which gives
10
- 7 10 disintegration’s^ per^ second^ ( dps ),^ i.e. ,1 c^ =
Activity of 1 g of Ra dps 226 10 3. 7 10
The millicurie ( mc ) and microcurie ( c ) are equal
to
3 10
and
6 10
curies i.e. 7
- 7 10 and 4
- 7 10 dps
respectively.
c mc c
3 6 1 10 10 ; c dps
10 1 3. 7 10
mc dps
7 1 3. 7 10 ; c dps 4 1 3. 7 10
(a) d,p type 7 3
6 3 Li ( d^ , p ) Li^ i.e. ,
1 1
7 3
2 1
6 3 Li^ H Li H
76 32
75 32 As (^ d , p ) As^ i.e. ,
1 1
76 32
2 1
75 32 As^ H As H
(v) Transmutation by - radiations
(a) , n type 8 4
9 4 Be (^ ^ , n ) Be^ i.e. ,
1 0
8 4
9 4 Be^ Be n
Synthetic elements : Elements with atomic
number greater than 92 i.e. the elements beyond
uranium in the periodic table are not found in nature
like other elements. All these elements are prepared by
artificial transmutation technique and are therefore
known as transuranic elements or synthetic elements.
Nuclear fission and Nuclear fusion
(1) Nuclear fission : The splitting of a heavier
atom like that of uranium – 235 into a number of
fragments of much smaller mass, by suitable
bombardment with sub-atomic particles with liberation
of huge amount of energy is called Nuclear fission.
Hahn and Startsman discovered that when uranium- 235
is bombarded with neutrons, it splits up into two
relatively lighter elements.
1 0
93 36
140 56
1 0
235 92 U^ n Ba Kr ^3 n^ +^ Huge^ amount^ of
energy
Spallation reactions are similar to nuclear fission.
However, they differ by the fact that they are brought
by high energy bombarding particles or photons.
Elements capable of undergoing nuclear fission
and their fission products. Among elements capable of
undergoing nuclear fission, uranium is the most
common. The natural uranium consists of three
isotopes, namely ( 0. 006 %)
234 U , ( 0. 7 %)
235 U and
238 U. Of the three isomers of uranium, nuclear
fission of
235 U and
238 U are more important. Uranium-
238 undergoes fission by fast moving neutrons while
235 U undergoes fission by slow moving neutrons; of
these two,
235 U fission is of much significance. Other
examples are
239 ^ Pu and
233 ^ U.
Uranium-238, the most abundant (99.3%) isotope
of uranium, although itself does not undergo nuclear
fission, is converted into plutonium-239.
239 92
1 0
238 92 U^ n U^ ;
0 1
239 92
239 92 U^ NP e
0 1
239 94
238 93 Np^ Pu e
Which when bombarded with neutrons, undergo
fission to emit three neutrons per plutonium nucleus.
Such material like U - 238 which themselves are non-
fissible but can be converted into fissible material ( Pu-
- are known as fertile materials.
Nuclear chain reaction : With a small lump of 235 U , most of the neutrons emitted during fission escape
but if the amount of 235 U exceeds a few kilograms
( critical mass ), neutrons emitted during fission are
absorbed by adjacent nuclei causing further fission and
so producing more neutrons. Now since each fission
releases a considerable amount of energy, vast
quantities of energy will be released during the chain
reaction caused by
235 U fission.
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.
Atomic pile or Nuclear reactor : It is a device to
obtain the nuclear energy in a controlled way to be used
for peaceful purposes. The most common reactor
consists of a large assembly of graphite (an allotropic
form of carbon) blocks having rods of uranium metal
(fuel). Many of the neutrons formed by the fission of
nuclei of
235 92 U escape into the graphite, where they
are very much slow down (from a speed of about 6000
or more miles / sec to a mile / sec ) and now when these
low speed neutrons come back into the uranium metal
they are more likely to cause additional fissions. Such a
substance like graphite, which slow down the neutrons
without absorbing them is known as a moderator.
Heavy water, D 2 O is another important moderator
where the nuclear reactor consists of rods of uranium
metal suspended in a big tank of heavy water
(swimming pool type reactor). Cadmium or boron are
used as control rods for absorbing excess neutrons.
Plutonium from a nuclear reactor : For such
purposes the fissile material used in nuclear reactors is
the natural uranium which consists mainly (99.3%) of
U - 238. In a nuclear reactor some of the neutrons
produced in U - 235 (present in natural uranium) fission
Uranium
E
3 E
9 E
Neutron
Fig. 7.
converts U - 238 to a long-lived plutonium isotope, Pu -
239 (another fissionable material). Plutonium is an
important nuclear fuel. Such reactors in which
neutrons produced from fission are partly used to carry
out further fission and partly used to produce some
other fissionable material are called Breeder reactors.
Nuclear reactors in India : India is equipped with
the five nuclear reactors, namely
(i) Apsara (1952) (ii) Cirus (1960)
(iii) Zerlina (1961) (iv) Purnima (1972) and
R - 5
Purnima uses plutonium fuel while the others utilize
uranium as fuel.
(2) Nuclear fusion : “Oposite to nuclear fission,
nuclear fusion is defined as a process in which lighter
nuclei fuse together to form a heavier nuclei. However,
such processes can take place at reasonable rates only
at very high temperatures of the order of several
million degrees, which exist only in the interior of
stars. Such processes are, therefore, called
Thermonuclear reactions (temperature dependent
reactions). Once a fusion reaction initiates, the energy
released in the process is sufficient to maintain the
temperature and to keep the process going on.
4 2 Energy Positron
0 1 Helium
4 2 Hydrogen
1 1 H ^ He e
This is not a simple reaction but involves a set of
the thermonuclear reactions, which take place in stars
including sun. In other words, energy of sun is derived
due to nuclear fission.
Controlled nuclear fusion : Unlike the fission
process, the fusion process could not be controlled.
Since there are estimated to be some
17 10 pounds of
deuterium( )
2 1 H in the water of the earth, and since
each pound is equivalent in energy to 2500 tonnes of
coal, a controlled fusion reactor would provide a
virtually inexhaustible supply of energy.
Hydrogen bomb : Hydrogen bomb is based on the
fusion of hydrogen nuclei into heavier ones by the
thermonuclear reactions with release of enormous
energy.
As mentioned earlier the above nuclear reactions
can take place only at very high temperatures.
Therefore, it is necessary to have an external source of
energy to provide the required high temperature. For
this purpose, the atom bomb, ( i.e. , fission bomb ) is used
as a primer, which by exploding provides the high
temperature necessary for successful working of
hydrogen bomb ( i.e. , fusion bomb ). In the preparation
of a hydrogen bomb, a suitable quantity of deuterium
or tritium or a mixture of both is enclosed in a space
surrounding an ordinary atomic bomb. The first
hydrogen bomb was exploded in November 1952 in
Marshall Islands; in 1953 Russia exploded a powerful
hydrogen bomb having power of 1 million tonnes of
TNT
A hydrogen bomb is far more powerful than an atom
bomb. Thus if it is possible to have sufficiently high
temperatures required for nuclear fusion, the deuterium
present in sea (as D 2 O ) is sufficient to provide all energy
requirements of the world for millions of years. The first
nuclear reactor was assembled by Fermi in 1942.
Difference between Nuclear fission and fusion
Nuclear fission Nuclear fusion
The process occurs only in
the nuclei of heavy elements.
The process occurs only in
the nuclei of light elements.
The process involves the fission of the heavy nucleus
to the lighter nuclei.
The process involves the fission of the lighter nuclei
to heavy nucleus.
The process can take place at
ordinary temperature.
The process takes place at
higher temperature
( 10 ) 8 C o .
The energy liberated during
this process is high
(200 MeV per fission)
The energy liberated during
the process is comparatively low
(3 to 24 MeV per fusion)
Percentage efficiency of the
energy conversion is comparatively less.
Percentage efficiency of the
energy conversion is high (four times to that of the
fission process).
The process can be controlled for useful
purposes.
The process cannot be controlled.
Application of radioactivity
Radioisotopes find numerous applications in a
variety of areas such as medicine, agriculture, biology,
chemistry, archeology, engineering and industry.
(1) Age determination : The age of earth has
been determined by uranium dating technique as
follows. Samples of uranium ores are found to contain
206 Pb as a result of long series of - and -decays. Now
if it is assumed that the ore sample contained no lead
at the moment of its formation, and if none of the lead
formed from
238 U decay has been lost then the
measurement of the 206 238 (^) Pb / U ratio will give the value
of time t of the mineral.
1 238
206
No.ofatomsof left
No. ofatomsof t e U
Pb
where is the decay constant of uranium- 238
Alternatively,
Amountof inthemineral presenttilldate
Initialamountof log
- 303 238
238
U
U t
Similarly, the less abundant isotope of uranium, 235 U eventually decays to 207 232 Pb ; Th decays to 208 Pb and
thus the ratios of
207 235 Pb / U and
208 232 Pb / Th can be used
to determine the age of rocks and minerals.
The accident of Chernobyl occurred in 1986 in USSR is
no older when radioisotopes caused a hazard there. The
nuclear radiations (alpha, beta, gamma as well as X -
rays) possess energies far in excess of ordinary bond
energies and ionisation energies. Consequently, these
radiations are able to break up and ionise the molecules
present in living organisms if they are exposed to such
radiations. This disrupts the normal functions of living
organisms. The damage caused by the radiations,
however, depends upon the radiations received. The
resultant radiation damage to living system can be
classified as,
(1) Somatic or pathological damage : This affects
the organism during its own life time. It is a permanent
damage to living civilization produced in body. Larger
dose of radiations cause immediate death whereas
smaller doses can cause the development of many
diseases such as paralysis, cancer, leukaemia, burns,
fatigue, nausea, diarrhoea, gastrointestinal problems
etc. some of these diseases are fatal. Many scientists
presently believe that the effect of radiations is
proportional to exposure, even down to low exposures.
This means that any amount of radiation causes some
finite risk to living civilization.
(2) Genetic damage : As the term implies,
radiations may develop genetic effect. This type of
damage is developed when radiations affect genes and
chromosomes, the body's reproductive material. Genetic
effects are more difficult to study than somatic ones
because they may not become apparent for several
generations.
The particle like mesons, positron, neutrino, etc,
about 20 in number are created by stresses in
nucleus but do not exist as component of nucleus.
Highest degree of radioactivity is shown by radium.
The nuclear forces are not governed by inverse
square law.
About 42 radioactive nuclides ( Z > 82) occur in
nature. Each of these gives stable end product of
an isotope of lead.
The half life is independent of physical or
chemical state of a radioactive element.
The average life of the natural radioactive element
vary from 10
It has been observed that fusion of 45 mg of
hydrogen produce as much energy as obtained from
one ton of coal.
Beryllium has been found to be the best moderator
as it occupies small space and have low absorption
cross-section.
The total life span of a radioactive element is
infinite.
The - radiation of total energy 1.02 MeV ,
emitted when a positron and an electron interact
are known as annilation radiation.
