Which of the following elements does not belong to actinoid series ?

Question

Accepted Answer

Option 3, Tm (Thulium), is the correct answer because it does not belong to the actinoid series. The actinoid series consists of elements with atomic numbers from 89 (Actinium, Ac) to 103 (Lawrencium, Lr). These elements are characterized by their filling of the 5f subshell.

1. **Explanation for Option 3 (Tm):**
   - Thulium (Tm) is an element with the atomic number 69. It belongs to the lanthanoid series, not the actinoid series. The lanthanoids are elements with atomic numbers from 57 (Lanthanum, La) to 71 (Lutetium, Lu), characterized by the filling of the 4f subshell. Therefore, Tm is correctly identified as not belonging to the actinoid series.

2. **Explanation for why other options are incorrect:**
   - **Option 1 (U):** Uranium (U) has an atomic number of 92 and is part of the actinoid series. It is well-known for its use in nuclear reactors and weapons. Uranium's presence in the actinoid series is due to its 5f subshell electron configuration.
   - **Option 2 (Np):** Neptunium (Np) has an atomic number of 93 and is also part of the actinoid series. It is a transuranic element that follows uranium in the periodic table and shares similar properties, including the filling of the 5f subshell.
   - **Option 4 (Fm):** Fermium (Fm) has an atomic number of 100 and is included in the actinoid series. It is a synthetic element that is part of the series due to its electron configuration involving the 5f subshell.

In summary, Option 3 (Tm) is the correct answer because Thulium is a lanthanoid, not an actinoid, while the other options (U, Np, Fm) are all elements that belong to the actinoid series.