How are radioactive decay series distinguished ? Name the decay series which is not natural but artifical.

Explanation

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