A radioactive decay series may be represented as shown:

To help understand what a decay series means, view the following demonstration.  
of a decay series 
What is a decay series?
The concept of a decay series may best be illustrated by an example,
like that of the uraniumradium series.
When uranium238
decays by alpha emission, thorium234 is formed.
Thorium234 is a beta emitter and decays to form protactinium234.
Protactinium234 in turn is a beta emitter and decays to form an isotope
of uranium, called uranium234.
Uranium234 decays by alpha emission to form thorium230, and this in
turn decays by alpha emission to yield the isotope radium226.
This process continues through five more alpha emissions and four more
beta emissions until it reaches the end product, an isotope of lead
(element 82), which is stable (nonradioactive).
The members of the series all share a common characteristic: their mass
numbers can be expressed by the simple formula 4n + 2, in which n is a
whole number.
What other series are there?
Other natural radioactive series are the thorium series, called the 4n
series because the mass numbers of all its members are exactly divisible
by four, and the actinium series, or 4n + 3 series.
The parent of the thorium series is the isotope thorium232, and the
actinium series begins with uranium235. A fourth series, the 4n + 1
series, all the members of which are artificially radioactive, has in
recent years been discovered.
Are radioactive decay series useful?
An interesting application of knowledge of radioactive elements is made
in determining the age of rocks.