Which of the following is not an actinoid?

Options

A. Curium (Z=96)
B. Californium (Z=98)
C. Uranium (Z=92)
D. Terbium (Z=65)

Answer

To determine which of the given elements is not an actinoid, we need to analyze the electron configurations of each element listed in the question.

### Step-by-Step Solution:

1. **Identify the Elements and Their Atomic Numbers:**
- Curium (Cm) - Atomic Number 96
- Californium (Cf) - Atomic Number 98
- Uranium (U) - Atomic Number 92
- Terbium (Tb) - Atomic Number 65

2. **Write the Electron Configurations:**
- **Curium (Cm)**: The electron configuration is [Rn] 5f^7 6d^1 7s^2.
- **Californium (Cf)**: The electron configuration is [Rn] 5f^10 6d^0 7s^2.
- **Uranium (U)**: The electron configuration is [Rn] 5f^3 6d^1 7s^2.
- **Terbium (Tb)**: The electron configuration is [Xe] 4f^9 6s^2.

3. **Identify Actinoids:**
- Actinoids are elements that have electrons filling the 5f subshell. This includes elements with atomic numbers from 89 (Actinium) to 103 (Lawrencium).

4. **Analyze the Electron Configurations:**
- Curium, Californium, and Uranium all have electron configurations that include the 5f subshell, which means they are actinoids.
- Terbium has an electron configuration that includes the 4f subshell, which means it is not an actinoid but a lanthanoid.

5. **Conclusion:**
- The element that is not an actinoid is **Terbium (Tb)**.

### Final Answer:
The element that is not an actinoid is **Terbium (Tb)**.