The radioactive elements have been classified into four types of series. These are
4
n
,
4
n
+
1
,
4
n
+
2
4
, and
4
n
+
3
4
series. Here
n
n
is a whole number obtained when the mass number of an element is divided by 4 and the other number
1
,
2
,
3
1
are remainders in respective series.
Thus, the number of each element of
4
n
4
series is completely divisible by 4. When the mass number of each member of
4
n
+
1
4
series is divided by 4, the remainder is
1
,
1
when the mass number of each element of
4
n
+
2
4
series is divided by 4, remainder is
2
,
2
when the mass number of each element of
4
n
+
3
4
series is divided by 4, the remainder is 3.
The neptunium
(
4
n
+
1
)
(
series consists of artificial elements while the other three series consists of naturally occurring elemens.
