Question

Which of the following elements belong to 4n-series?
A. Pb-207
B. Bi-209
C. Pb-208
D. Pb-206

Answer 1

The mass number of the members may be expressed as four times an appropriate integer (n) plus the constant for that series; thus, the thorium series is sometimes called the 4n series.

Lead $(Pb - 82)$ has four stable isotopes they are $Pb - 204,\,Pb - 206,\,Pb - 207,\,Pb - 208$. Lead-204 is entirely a primordial nuclide and is not a radiogenic nuclide. The three isotopes lead-206, lead-207, lead-208 represent the ends of three decay chains, the neptunium series, the actinium series, and the thorium series respectively.

Complete Step-by-Step Solution:
The 4n chain of Th-232 is commonly called the thorium series. Beginning with naturally occurring thorium-232, this series includes the following elements actinium, bismuth, lead, polonium, radium, radon and thallium. All are present, at least transiently, in any natural thorium-containing sample whether metal, compound or mineral. The series ended with lead-208.

This radioactive decay chain consists of unstable heavy atomic nuclei that decay through a sequence of alpha and beta decays until a stable nucleus is achieved. In case of thorium series, the stable nucleus is lead-208.

Science alpha decay represents the disintegration of the parent nucleus to daughter through the emission of the nucleus of a helium atom; there are only four decay series. Within each series, therefore the mass number of the member may be expressed as four times an appropriate integer(n) plus the constant for the series. As a result, thorium series is known as the 4n-series.
$Pb - 208$ belongs to 4n-series. Also called thorium series, in 4n-series, mass number divides completely by 4.
Thorium on reaction with -$6\alpha$and -$4\beta$ gets converted into lead (Pb).

So, therefore the correct answer is C. i.e. Pb-208.

Note: The 4n chain of Th-232 is commonly called as the thorium series
In case of thorium series, the stable nucleus is lead-208. Within each series, therefore the mass number of the member may be expressed as four times an appropriate integer(n) plus the constant for the series. As a result, thorium series is known as the 4n-series.