Question

There are $14$ elements in actinoid series. Which of the following elements does not belong to this series?

• A. $Fm$
• B. $Np$
• C. $U$
• D. $Tm$

Answer 1

The correct option is D $Tm$

Actinoid series mainly includes $14$ elements from atomic number $90$ to $103.$

Among the given elements, the atomic number of $Tm\left(\text{Thulium}\right)$ is $69,$ which signifies that this element does not belong to the actinoid series.

The atomic numbers of $U,Np$ and $Fm$ are $92,93$ and $100$ respectively. Hence, they all belong to the actinoid series.

So, option $\left(C\right)$ is the correct answer.