### 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-

_{92}U^{238} —————————> _{82}Pb^{206} + 8 _{2}He^{4} + 6 _{-1}e^{0}

**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-** ** _{90}Th^{234}** 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_{90}Th^{232}and ends with_{82}Pb^{208}.**(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_{93}Np^{237}and ends with_{83}Bi^{209}.**(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_{92}U^{238}and ends with_{82}Pb^{206}.**(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_{92}U^{235}and ends with_{82}Pb^{207}.