Radioactive Disintegration Series:
The emission of α or β-particles from a Radioactive element results in the formation of a new element known as Daughter element and if it has an unstable nucleus, it further disintegrates to produce a new daughter element and the process continues till the end product is an element with a stable nucleus. Such a series of spontaneous radioactive changes starting from the parent element with an unstable nucleus up to the formation of a stable nucleus is called Radioactive Disintegration Series. The first natural radioactive series to be discovered is Uranium-238 series which involve 14 steps, out of which 8 involve α-emission and 6 involve β-emission. The overall process can be written as-
92U238 —————————> 82Pb206 + 8 2He4 + 6 -1e0
Rutherford and Soddy observed that disintegration of natural radioactive elements could be classified into four series known as Thorium, Neptunium, Uranium and Actinium series. The change in mass number of Parent and daughter nuclei (a nucleus of a given mass number and the atomic number produced in a nuclear reaction is known as Nuclide. Example- 90Th234 is Th-234 nuclide in any series will always occur by 4 units. When a mass number of any nuclide of U-238 series divided by 4, the remainder is always 2. It means that a mass number of various nuclides of U-series can be represented by (4n + 2), where ‘n’ is an integer. On this basis, the various radioactive series are as follows-
- 4n Series- This is also called Thorium Series and mass number of each nuclide of the series is divisible by 4. It starts with 90Th232 and ends with 82Pb208.
- (4n + 1) Series- This is also called Neptunium Series and mass number of each nuclide of the series gives a remainder of 1 when divided by 4. It starts with 93Np237 and ends with 83Bi209.
- (4n + 2) Series- This is also called Uranium Series and mass number of each nuclide of the series gives a remainder of 2 when divided by 4. It starts with 92U238 and ends with 82Pb206.
- (4n + 3) Series- This is also called Actinium Series (which is one of the most important disintegration products of this series) and mass number of each nuclide of the series gives a remainder of 3 when divided by 4. It starts with 92U235 and ends with 82Pb207.
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