Nuclear chemistry
A reaction in which nucleus of an atom undergoes a change is called nuclear reaction and the branch of chemistry which deals with the study of nuclear reactions is called Nuclear chemistry.
Radioactivity
There are certain elements like Uranium, Radium etc. And their salts which send out mysterious radiations which are more penetrating than X-rays. These radiations affect photographic plate and also ionise gases.
“The phenomenon of spontaneous emission of certain invisible radiations by elements like uranium is called radioactivity.
The radiations emitted are called radioactive radiations and the elements which emit such radiations are called Radioactivity.”
The radiations emit three types of radiations, called (I) α-particle (ii) β-particle and (iii)y (gamma) rays.
Group displacement law
(i)                  When a α-particle is emitted from a radioactive element, a daughter element is formed which atomic number less by four units as has compared to the parent element. e.g.  consider when uranium loses α-particle
(ii)                When a β-particle is emitted from a radioactive element, the daughter element formed has its atomic number raised by one. For example, consider when thorium loses a β-particle.
Group displacement law is stated as: “In α-ray change, the new element obtained has mass number less by four units and it occupies a position two columns to the left of the parent element in the periodic table. In β-ray change, the new element has the same mass number and occupies position in the periodic table one column to its right.”
Half life period (t1/2)
Half life period for radioactive decay is the time required for the disintegration of one half of the radioactive species initially present.
We know that for radioactive disintegration:  λ=2.303/tlogN0/N
Let t1/2 be the time in which the number of atoms will be half of the number of atom in the beginning. Thus, in time t1/2; N=N0/2.
Substitution in the above relation:
λ=2.303/t1/2logN0/N0/2 or  t1/2=2.303/t log2=2.303/λ90.3010)
 Hence, half life period, t1/2= 0.693/λ
Important. Like the first order reaction, the half life period of the radioactive element is independent of the amount of the substance taken i.e. the initial amount of the substance.
Significance of half life period
  1.    The half life period is constant for each radioactive element. From this value, it is possible to               characterise an element.
  2.      The relative stabilities of radioactive isotopes can be compared from their half lives. Longer                  the half life more is the stability of the element.

Unit of radioactivity.
(a)    Curie. It is amount of the radioactive substance which gives 3.7×1010 disintegrations per second.
(b)   Rutherford. It is the amount of the radioactive substances which gives 106 disintegrations per second.






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