Which of the following element belong to actinide series ?

Options

A. La
B. Gd
C. LU
D. Th

Answer

To determine which of the given elements belongs to the actinide series, we need to understand the characteristics of the actinide series and the electronic configurations of the elements provided.

### Step-by-Step Solution:

1. **Understand the Actinide Series**:
- The actinide series consists of 15 elements from actinium (Ac, atomic number 89) to lawrencium (Lr, atomic number 103).
- In this series, the 5f subshell is progressively filled with electrons.

2. **Identify the Elements**:
- The elements provided in the question are lanthanum (La), gadolinium (Gd), lutetium (Lu), and thorium (Th).

3. **Check Each Element**:
- **Lanthanum (La)**:
- Atomic number: 57
- Electronic configuration: [Xe] 5d¹ 6s²
- Conclusion: Lanthanum is sometimes considered a d-block element and does not belong to the actinide series.

- **Gadolinium (Gd)**:
- Atomic number: 64
- Electronic configuration: [Xe] 4f⁷ 5d¹ 6s²
- Conclusion: Gadolinium is part of the lanthanide series and does not belong to the actinide series.

- **Lutetium (Lu)**:
- Atomic number: 71
- Electronic configuration: [Xe] 4f¹⁴ 5d¹ 6s²
- Conclusion: Lutetium is also part of the lanthanide series and does not belong to the actinide series.

- **Thorium (Th)**:
- Atomic number: 90
- Electronic configuration: [Rn] 6d² 7s²
- Conclusion: Thorium is the first element of the actinide series.

4. **Final Answer**:
- The element that belongs to the actinide series is **Thorium (Th)**.