Decay series:

  (I) The Thorium Series: This series begins with and ends with
The mass numbers (sum of neutrons and protons numbers) in the series are given by 4 n , where, n is an integer. That is way this series is known as 4 n series.

The Thorium Series (4 n )

(II) The Neptunium Series: This series begins with and ends with
The mass numbers (sum of neutrons and protons numbers) in the series are given by 4 n + 1, where, n is an integer. That is way this series is known as 4 n + 1 series.

The Naptunium Series (4 n + 1)