By Sunil Bhardwaj

8672 Views


At the different timings, different model of the nucleus is proposed. One of the important model is Liquid Drop Model. The model was proposed by Neils Bohr and Wheeler in 1937 and independently by Frenkel. They treat the nucleus as a homogeneous entity consist of certain no. of protons and neutrons..

Similarities:
1) As a drop of the liquid is consist of large number of molecules which are present in the same volume, similarly nucleus is made-up of large number of protons and neutrons present in the same nuclear volume.
2) As the drop of liquid is homogeneous in density, charge etc. same is for the nucleus.
3) As a drop of liquid cannot be compressed, similarly nucleus also cannot be compressed.
4) In the case of drop of the liquid, the forces between all the molecules is same. Similarly nuclear forces in all the nucleons is nearly same.$$ f\left( p-p \right) \approx f\left( p-n \right) \approx f\left( n-n \right) $$ 5). When a very little amount of liquid is added to a drop. The drop of the liquid grows in size. Similarly projectiles (Light Particles) can be added to the nucleus forming the compound nucleus. $$ _{ 7 }^{ 14 }{ N }+_{ 2 }^{ 4 }{ He }\longrightarrow _{ 9 }^{ 18 }{ F } $$ 6) Small drops of the liquid combine to form a big drop of the liquid. Similarly small nuclei combine forming big nucleus. This is known as nuclear fusion phenomenon. $$ 4 \ _{ 1 }^{ 1 }{ H }\longrightarrow _{ 2 }^{ 4 }{ He }+2{ \beta }^{ + } + Energy $$ 7) A very big drop of the liquid breaks up into smaller droplets. Similarly heavy nucleus breaks up into smaller. This phenomenon is called nuclear fission. $$ _{ 92 }^{ 235 }{ U } + _{ 0 }^{ 1 }{ n }\longrightarrow _{ 56 }^{ 144 }{ Ba }+ _{ 36 }^{ 89 }{ Kr } + 3_{ 0 }^{ 1 }{ n } + \gamma - ray + Energy $$ 8) As in a drop of liquid evaporation phenomenon take place. Similarly in the nuclear reaction emission of the nucleons take place. For example The Atomic transmutation reaction, the particles are ejected out.
9) As in a drop of the liquid the molecules are moving continuously due to thermal motion of the molecules. Similarly in the nucleus, the nucleons (Protons and Neutrons) are moving because of K.E.

Merits:
1) It explain the phenomenon of nuclear fission.
2) It explain the nuclear fusion phenomenon.
3) It helps in calculating binding energy of the nuclei.
4) It predicts the emission of ?, ? and ? - rays.
5) It support the Bohr’s theory of the formation of compound nucleus.

Limitations:
1) This model cannot explain the extra stability of the nuclei having the number of protons and neutrons 2, 8, 20, 28, 50, 82 and 126 i.e. inert gases.
2) The model is true only for elements having mass number, between 20 and 150 (20 - 150).