Radioactive Disintegration Series

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.