Which of the following is not an actinide ?

Options

A. Uranium
B. Curium
C. Californium
D. Erbium

Answer

To determine which of the following elements is not an actinide, we first need to understand what the actinide series is. The actinides are a series of 15 chemical elements in the periodic table that range from atomic number 89 (actinium) to atomic number 103 (lawrencium).

### Step-by-Step Solution:

1. **Identify the Range of Actinides**:
The actinide series includes elements with atomic numbers from 89 (Actinium) to 103 (Lawrencium).

2. **List the Elements and Their Atomic Numbers**:
- Uranium (U) - Atomic number 92
- Curium (Cm) - Atomic number 96
- Californium (Cf) - Atomic number 98
- Erbium (Er) - Atomic number 68

3. **Check Each Element Against the Actinide Range**:
- **Uranium (92)**: This is within the actinide range (89-103).
- **Curium (96)**: This is also within the actinide range (89-103).
- **Californium (98)**: This is again within the actinide range (89-103).
- **Erbium (68)**: This is outside the actinide range (89-103).

4. **Conclusion**:
Since Erbium (atomic number 68) does not fall within the range of actinides, it is the element that is not an actinide.

### Final Answer:
The element that is not an actinide is **Erbium (Er)**.

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