Which of the following is not an actinoid?

Options

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

Explanation

To determine which of the following elements is not an actinoid, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Definition of Actinoids**:
- Actinoids are a series of elements in the f-block of the periodic table. They are characterized by the filling of the 5f orbitals.

2. **Identify the Range of Actinoids**:
- The actinoids have atomic numbers ranging from 90 to 103. This means any element with an atomic number within this range is considered an actinoid.

3. **List the Given Options**:
- The options provided are:
- Curium (Cm) - Atomic number 96
- Californium (Cf) - Atomic number 98
- Uranium (U) - Atomic number 92
- Terbium (Tb) - Atomic number 65

4. **Check Each Option Against the Actinoid Range**:
- Curium (96): Falls within the range (90-103) → Actinoid
- Californium (98): Falls within the range (90-103) → Actinoid
- Uranium (92): Falls within the range (90-103) → Actinoid
- Terbium (65): Does not fall within the range (90-103) → Not an actinoid

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

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

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