Nuclear Chemistry-Introduction Theory of radioactive disintegration Mass Defect,Liquid Drop Model

1. NUCLEAR CHEMISTRY
Shafna Jose
Assistant Professor
Department Of Chemistry
St. Mary’s College
Thrissur

2. INTRODUCTION
• Branch of chemistry deals with the of study of nuclear particles, nuclear forces
and nuclear reactions.
• Nucleus contains positively charged protons and electrically neutral neutrons
collectively known as nucleons.
• Atomic number : number of protons present in the nucleus.
• Mass number : The sum of protons and neutrons in a nucleus.
Nuclear Chemistry, Shafna Jose, St. Mary’s College.

3. • Each nuclide is defined by the number of protons ‘Z’ and
number of neutrons 'N’ that it’s nucleus contains.
• The general representation of a nuclide is given as
A X Z
X – denotes chemical symbol
Z – Atomic number( number of protons )
A – Mass number (number of neutrons + number of protons )
• Examples are 16O8,
31P15,
238U92 , etc.
Nuclear Chemistry, Shafna Jose, St. Mary’s College.

4. Theory of radioactive disintegration :
• Atoms of all radioactive elements undergo spontaneous disintegration with the emission of α or
β particles.
• Modes of decay : α –emission or β- emission.
α –emission : When a radionuclide decays by emitting an α particle, the atomic number decreases
by 2 units and mass number increases by 4 units.
Examples are :
238U92 → 234 Th90 + 4He2
215 Po 84 → 211 Pb82 + 4He2
Nuclear Chemistry, Shafna Jose, St. Mary’s College.

5. β- emission : When a radionuclide decays by emitting an β- particle atomic number
increases by 1 unit and on change in mass number.
Examples are :
234 Th 90 → 234Pa91 +
0e-1
211 Pb 82 → 211Bi83 + 0e-1
• The increase in nuclear charge and emission of electron from the nucleus takes place
due to the conversion of a neutron into a proton and an electron during the process.
• The γ rays emitted as a secondary effect of α- or β- emission.
Nuclear Chemistry, Shafna Jose, St. Mary’s College.

6. Mass Defect
• It is equal to the mass lost as an equivalent amount of energy during the formation of a given nucleus
from the component nucleons.
• If M, mp and mn are the masses of the nucleus (AXZ), a proton and a neutron, respectively, then
∆m = Zmp + Nmn – M = Zmp + (A – Z)mn - M
Binding Energy
• Energy released during the formation of a nucleus from it’s constituent nucleons.
• If ∆m is the mass defect , then binding energy , B.E = ∆mc2 .
B.E = [ Zmp + (A – Z)mn – M]c2
Nuclear Chemistry, Shafna Jose, St. Mary’s College.

7. Liquid Drop Model
Bohr tried to compare properties of a nucleus with a drop of a liquid and found many
similarities. Some of them are given below.
• Liquid drop has a large no: of molecules just like a nucleus has large no: of nucleons.
• Both the liquid drop and nucleus are homogenous and incompressible.
• Force between all the nucleons is same. Nuclear force is independent of charge and spin.
Nuclear Chemistry, Shafna Jose, St. Mary’s College.

8. • Heat of vaporization of a liquid corresponds to binding energy of nucleons in a
nucleus.
• Evaporation of a liquid corresponds to radioactive emission from a radioactive
isotope.
• In both liquid drop and nucleus, intermolecular forces are short range forces.
• By capturing high energy particle from outside nucleus form a compound nucleus as
does a liquid drop , to get excited.
Nuclear Chemistry, Shafna Jose, St. Mary’s College.

9. • Compound nucleus or the liquid drop may get de-excited by the following changes.
• Compound nucleus
a) By emission of radiation
b) By emission of a nucleon
c) By nuclear fission
• Liquid Drop
a)By cooling
b)By evaporation
c)By breaking up into droplets
• Two liquid drops undergo fusion and form a bigger drop, similarly lighter nuclei undergo
fusion and form a bigger nucleus.
Nuclear Chemistry, Shafna Jose, St. Mary’s College.

10. Nuclear Fission
• Splitting of a heavy nucleus into two fragments when bombarded with a suitable sub
atomic particle with the simultaneous release of a huge amount of energy.
235U92 + 0n1 → 144Ba56 + 20n1 + Energy
Fissile nuclides : undergo fission on bombardment with thermal or slow neutrons.
Eg: 239Pu94,
233U92
Fissionable nuclides : Require fast neutrons to produce their fission
Eg: 231Pa91,
238U92.
Critical mass : The minimum amount of the target material required to sustain a fission
chain reaction at a constant rate
Nuclear Chemistry, Shafna Jose, St. Mary’s College.

11. Nuclear Fusion
• Process in which lighter nuclei fuse together to form a heavier nucleus with the simultaneous
release of a huge amount of energy.
1H3 + 1H1 → 4He2 + 20 MeV
3H1 + 2H1 → 4He2 +0n1 + 17.8 MeV
• Fusion requires high temperature.
• These reactions are also called thermonuclear reactions.
• The function of a hydrogen bomb is based on nuclear fusion which is initiated by energy
from a fission bomb.
• Fusion reactions are common in the interior of stars.
Nuclear Chemistry, Shafna Jose, St. Mary’s College.

12. Nuclear Reactions
• It involves changes in the number of nucleons present in the nucleus.
• Nuclear reactions leads to atomic transformations or transmutations.
• Nuclear reactions involve energies a million times greater than those involved in
chemical reactions.
• Nuclear reactions may be divided into two categories :
a) radioactivity
b) artificial radioactivity
Nuclear Chemistry, Shafna Jose, St. Mary’s College.

13. Radioactivity :
• The nucleus undergoes a spontaneous change, i.e., a change by itself.
Artificial radioactivity :
• The reaction is brought about artificially through the interaction of two lighter nuclei.
•
• This is done by bombarding a relatively heavier nucleus with a lighter nucleus.
Induced radioactivity :
• The products in a nuclear reaction are a heavy nucleus and a proton, a neutron, an α
particle or a γ - ray photon.
• If the heavy nucleus is unstable it may start disintegrating like the natural radioactive
elements.
Nuclear Chemistry, Shafna Jose, St. Mary’s College.

14. Q Values of Nuclear Reactions
• The complete equation for a nuclear reaction include the energy change of
the reaction as well.
• It is denoted by Q.
• Rutherford’s transmutation reaction can be represented as
7N14 + 4He2 → 17O8 + 1H1 + Q
where Q is the nuclear reaction energy.
• Q is +ve if the reaction is exoergic.
• Q is –ve if the reaction is endoergic.
Nuclear Chemistry, Shafna Jose, St. Mary’s College.

15. References :
Physical Chemistry – Puri, Sharma, Pathania.
Essentials of Nuclear Chemistry – H.J. Arnikar
Nuclear Chemistry, Shafna Jose, St. Mary’s College.