Nuclear Chemistry 20-10-2020
Nuclear Chemistry 20-10-2020
Nuclear Chemistry 20-10-2020
by radioactive nuclei.
2. Protons: The nucleus of ordinary hydrogen isotope (Protium) was named as
proton by E.Rutherford. They have one unit of positive electronic unit and
mass is equal to 1.67 10 27 kg . They are denoted by p, 11H.
3. Neutrons: It was first discovered by James Chadwick during bombardment of
beryllium nuclei with fast moving p a r tic le s . These are chargeless particles
and of mass slightly more than that of proton. They are denoted by n or 01 n .
While the electron and the proton are stable particles capable of
independent existence, the neutron is unstable particle when present
outside the nuclei and disintegrate as
Neutron( 01 n) Proton( 11 H) Electron( 01 e) Neutrino( 00 n )
However, neutrons are captured by atomic nuclei much faster than they
have a chance to decay and form its isotope.
4. Positrons: Positrons are the antiparticles of electron, They have one unit of
positive electronic charge and the same mass as electron. They are denoted
by 0
1 e or 1 . They are discovered by Carl D. Anderson in 1932 during
studies on cosmic rays.
When a particle and its antiparticle meet, both are annihilated (destroyed
completely) and their mass gets converted into an energy form such as -
radiation. The energy equivalent produced is given by Einstein’s mass-
energy equation (E mc2 ). Positrons, though they are stable non-
disintegrating particles but found to exist not more than a micro-second as
our surroundings are rich in electrons.
A number of other fundamental particles like antiproton, neutrino and
mesons have also been discovered. Photons are also sometimes treated as
fundamental particles as they can be assigned momentum while in motion.
NUCLEAR PROPERTIES OF ELEMENTS
Nuclear size: Rutherford scattering experiment indicated that atomic nuclei
were extremely small and were possibly small than 1012 cm in diameter.
But, neutron scattering experiment is more helpful in estimating nuclear
radii and general relationship has been found between for nuclear radii, R
and nuclear mass numbers, A.
1
R R0 A 3
where R 0 1.33 10 13 cm and is a common constant for all nuclei.
Nuclear shape: A perfectly spherical nucleus will exert a uniform force of
attraction in all directions on the electrons surrounding it. A non-spherical
nuclear shape, and therefore a non-spherical distribution of protons in the
nucleus, gives rise to the property of nuclear quadrupole moments. Thus,
one another and their separation needs much stronger force. Neutrons
behave similarly towards protons and other neutrons in the matter of
increase in the forces of attraction with decreasing distances. However, do
not show any initial repulsion.
Mass defect
For all nuclei it is observed that the observed atomic mass of all known
isotopes (except hydrogen) is always less from the sum of the masses of
protons and neutrons present in it. This difference between expected mass
(calculated by adding the masses of protons and neutrons) and the actual
mass of the isotope is called mass defect. It is denoted by m and expressed
in atomic mass units.
Consider 24 He nuclei, it has 2 protons and 2 neutrons. Its expected mass is
Expected mass (2 1 .0 0 7 5 8 2 1 .0 0 8 9 3 )
4 .0 3 3 0 2 a .m .u
Mass defect, m 4 .0 3 3 0 2 4 .0 0 3 9 0
m 0 .0 2 9 1 2 a .m .u
Binding energy
The mass defect is converted into energy which is released in the formation
of the nucleus. The amount of energy which released during the formation
of a nucleus from its protons and neutrons is called binding energy. The
same of energy is required to separate the nucleons.
Binding energy is calculated according to Einstein mass-energy relation,
B.E mc2
14.94 10 11
B.E 931.5 MeV
1.604 10 19
B .E m 9 3 1 .5 M e V
As the mass defect increases binding energy of the nucleus increases. But,
the stability of a nucleus can be determined when binding energy is divided
by the number of nucleons, this is called binding energy per nucleon or
mean binding energy. The average binding energy for most of the stable
nuclei is around 8 MeV. Iron has the maximum average binding energy
(8.79 MeV) and its nucleus is thermodynamically most stable.
Packing fraction
The difference of actual isotopic mass and the mass number is defined in
terms of packing fraction.
Actual isotopic mass Mass number
Packing fraction 104
Mass number
Packing fraction can be positive, negative or even zero. Carbon ( 12
6 C ) has zero
protons i.e., n 1.
p
radioactive or unstable.
Some general rules for predicting how an unstable nucleus may
change to acquire stability can be stated as follows.
1. The unstable nuclei have an excess of neutrons which lies above the
stability belt, this excess may be reduced by
(i) Emission of neutrons
(ii) Emission of - particles or electrons
Neutron Proton
2. The unstable nuclei have less number of neutrons which lies below
the stabiliyt belt, this imbalance may be reduced by
(i) Emission of -particles or positrons
Proton Neutron
are emitted.
RADIOACITIVITY
Whenever an unstable nucleus becomes stable one, energy is always
released; mass particles may or may not be emitted at the same time. When
these particles are emitted they come off with high speed ranging from a few
thousand kilometers per second to about 95 % of the velocity of light.
Transmutation of elements
In a radioactive process nuclei of atoms of one kind change into nuclei of
atoms of another element. The changed nuclei readjust the number of
revolving electrons to become electrically neutral. This change of elements
through alterations in atomic nuclei is called transmutation of elements.
Some of such particle-nucleus interactions are
By particles
14 4 17 1
7N 2 He 8O 1p (, p p ro c es s )
By neutrons
113 1 114
48 Cd 0n 48 Cd (n, p ro cess )
By protons
7 1
3 Li 1p 84 Be ( p, p r o c e s s )
Artificial radioactivity
When the nuclei of some elements on bombardment of fast moving or
accelerated particles like neutrons, protons etc., form a new unstable
nuclei, which immediately undergo transformation to more stable nuclei by
emitting particles and photons or just photons is known as induced or
artificial radioactivity of the parent nuclei.
Ex: Bombardment of aluminium metal sheet by particles yields
phosphorus-30 nuclei and neutrons. This phosphorus-30 is unstable and
spontaneously undergoes transformation to silicon-30 by emitting positron.
27 4 30 1
13 Al 2 He 15 P 0n
30
15 P 30
14 Si
TYPES OF RADIATIONS
The radioactive radiations are of three types. These were sorted out by
Rutherford (1902) by passing them between two oppositely charged plates.
The one bending towards the negative plate carried positive charge and were
named -(alpha) rays. Those bending towards the positive plate and
carrying negative charge were called -(beta) rays. The third type of
radiation, being uncharged, passed straight through the electric field and
were named -(gamma) rays. , and -rays could be easily detected as they
cause luminescence on the zinc sulphide screen placed in their path.
On emitting a -particle, the daughter element lies one position to the left
and there is no change in mass number. After ejection of a neutron position
of element in the table remains the same but the mass number decreases by
one.
Z2 Z 1 Z Z 1
A
Z 1 D3
A 4 A
Z 2 D2 Z 1 D1
A
ZP
n
A 1
Z D4
RADIOACTIVE SERIES
The whole series of elements starting with the parent radioactive element to
the stable end-product is called a Radioactive Disintegration Series.
CALCULATION OF HALE-LIFE
dN
we can write dt
N
On integration,
dN
dt or –ln N = t + K(constant)
N
ln N t lnN0 or N
ln 0 t
N
N0
At half-life time t1 , N
2 2
N
2.303 log 0 2.303 log 2 t 1
N0 2
2
0693
t1 .
2
UNITS OF RADIOACTIVITY
Radioactivity of radium is taken as a standard for measuring radioactivity
equivalent weights or equivalent quantities of different radioactive
substances. These weights are measured in curie units, C. One curie of
radium is taken as one gram of it. In one gram radium, on an average
3.7 1010 atoms disintegrate per second. So one curie of any radioactive
substances is such a quantity of it which gives 3.7 1010 atomic
disintegrations per second.
Rutherford is a more recent unit.
1 Rutherford = 106 dps
The S.I. unit is Becquerel
1 Bq = 1 dps
Since exposure to radioactive emanations is harmful, physiologists use a
unit for measuring the exposure dose. This is called a roentgen, r, and is
defined as the quantity of X or radiations which produces 1.611012 ion
pairs in 1 gram of air. This is equivalent to absorption of 84 ergs of energy
per gram of air.
Another radiation unit used is a rad. The rad is the dosage of any nuclear
emanation (particles or photons) equivalent to the absorption of 100 ergs of
energy per gram of any material.
AVERAGE LIFE
In a radioactive substance, some atoms decay earlier and others survive
longer. The statistical average of the lives of all atoms present at any time is
called the Average life. It is denoted by the symbol and has been shown to
be reciprocal of decay constant .
1
t 1 of B
NA
2
NB t 1 of A
2
Thus the atoms of A and B are present in the ratio of their half-lives.
The radioactive equilibrium differs from a chemical equilibrium in that it is
irreversible.
RADIOACTIVE DATING
The age of an old piece of wood can be determined by radioactive dating
technique. The atmosphere contains radioactive carbon dioxide, 14
CO2 , and
both types of carbon dioxide and converts them to carbon-14 and carbon-12
photosynthesis. Thus a living plant contains radioactive carbon-14 and
stable carbon-12 in a fixed ratio. When the plant dies, the uptake of carbon
from the atmosphere stops. Hence, carbon-12 remains unchanged but
carbon-14 decays by -emission.
14
6 C 14
7 N 01 Half life 5730 years
radioactive
12
6 C No change
stable
14
C
As a result, 12
decreases with lapse of time.
C
6. At the end of the reaction nuclear No nuclear waste is left at the end
waste is left behind. of fusion reaction.