Which of the following is not an actinoid ?

Options

A. Cerium
B. Californium
C. Uranium
D. Terbium

Explanation

To determine which of the following elements is not an actinoid, we need to analyze the given options based on their atomic numbers and electronic configurations. The actinoids are a series of 15 elements with atomic numbers ranging from 89 to 103. Their general electronic configuration is typically represented as:

\[ \text{Rn} \, 5f^{1-14} \, 6d^0-1 \, 7s^2 \]

Now, let's evaluate the options provided:

1. **Cerium (Ce)**:
- Atomic Number: 58
- Electronic Configuration: \[ \text{Xe} \, 4f^1 \, 5d^1 \, 6s^2 \]
- Since 58 is outside the range of 89 to 103, Cerium is **not** an actinoid.

2. **Californium (Cf)**:
- Atomic Number: 98
- Electronic Configuration: \[ \text{Rn} \, 5f^{10} \, 7s^2 \]
- Since 98 falls within the range of 89 to 103, Californium is an actinoid.

3. **Uranium (U)**:
- Atomic Number: 92
- Electronic Configuration: \[ \text{Rn} \, 5f^3 \, 6d^1 \, 7s^2 \]
- Since 92 is also within the range of 89 to 103, Uranium is an actinoid.

4. **Terbium (Tb)**:
- Atomic Number: 65
- Electronic Configuration: \[ \text{Xe} \, 4f^9 \, 6s^2 \]
- Since 65 is outside the range of 89 to 103, Terbium is **not** an actinoid.

From the analysis, both Cerium (Ce) and Terbium (Tb) are not actinoids. However, since the question asks for one option, Cerium is the first element listed that is not an actinoid.

**Final Answer**: Cerium (Ce) is not an actinoid.