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 following elements is not an actinoid, we need to understand the classification of elements in the periodic table, specifically focusing on the actinoids and lanthanoids.

### Step-by-Step Solution:

1. **Understand Actinoids**: Actinoids are elements that belong to the 5f series of the periodic table. They are located in the f-block and include elements that follow actinium (Ac, atomic number 89). The actinoids range from atomic number 89 (actinium) to 103 (lawrencium).

2. **List the Given Options**: The options provided are:
- A) Curium (Cm, atomic number 96)
- B) Californium (Cf, atomic number 98)
- C) Uranium (U, atomic number 92)
- D) Terbium (Tb, atomic number 65)

3. **Identify Actinoids in the Options**:
- Curium (Cm) is an actinoid because it is part of the 5f series.
- Californium (Cf) is also an actinoid as it is part of the 5f series.
- Uranium (U) is an actinoid as well, being part of the 5f series.
- Terbium (Tb), however, is not an actinoid; it is a lanthanoid, which belongs to the 4f series.

4. **Conclusion**: Based on the classification, the element that is not an actinoid among the options is Terbium (Tb).

### Final Answer:
**D) Terbium (Tb)** is not an actinoid.

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