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Chapter: 11th 12th std standard Class Organic Inorganic Physical Chemistry Higher secondary school College Notes

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Nuclear Chemistry - Indroduction

Radioactivity , Half life period, Binding energy of Nucleus, decay series, decay constant, radioactive reactions formula

Nuclear Chemistry

 

Radioactivity : The phenomenon of spontaneous disintegration of certain disintegration of certain atomicnucleiresultingintheemissionofradioactiveraysiscalledradioactivity. Radioactivity is a nuclear phenomenon and it is not affected by external factors such as temperature, pressure etc. This phenomenon was discovered by Henry Becqurel.

 

To explain the spontaneous decay of radioactive elements, Rutherford and Soddy put forward the theory of radioactive disintegration. According to this theory the quantity of a radioactive element which disappears in unit time is directly proportional to the amount (atoms) of radioactive substance present at that time.

Based on the above theory, the following equation is derived which confirms that all radioactive reactions follow I order

 

t = ( 2.303 / l   )  log(N0)/N)

l = decay constant

N0= Number of radioactive atoms present initially

N= Number of radioactive atoms at time 't'

 

Half life period : The time required to disintegrate one half of any radioactive substance is called half life period (t). The half life period (t) of a radioactive substance is independent of initial concentration. It depends only on the disintegration constant (l) of the radioactive element. t is used to indicate the relative stability of radioactive substance. If t is the shorter, faster is the rate of decay and hence the substance is more unstable and viceversa.

 

t  = 0.693 / l

Average life, t (Tau) = 1/ l = t  / 0.693  = 1.44t

*    Since radioactivity is a nuclear phenomenon, it must be connected with the  instability of the nucleus.

*    An a - particle is equal to the bundle of two protons and two neutrons and  hence it is equal to the Helium nucleus (2He4).

*    b-particle is a fast moving electron.

*    g-radiation is a waver of very short wavelength with very high energy.

*    Radioactive decay series : Radioactive heavy nuclei decay by a series of a - emission or b emissions, finally resulting in the formation of a stable isotope of lead. There are about 4 decay series.

4n - Thorium series

4n+1 - Neptunium series

4n+2 - Uranium series 4n+3 - Actinium series

*    Binding energy of Nucleus

Whenever a nucleus is formed, certain mass is converted into energy. Hence for atom, the atomic mass is lower than the sum of masses of protons, neutrons and electrons present. The difference in mass is termed as "mass defect". This is the measure of the binding energy of proton and neutron in the nucleus. The relationship between mass - energy is explained by Einstein equation DE = Dm C2.

 

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