Which of the following belongs to the category of transition metal?

Options

A. K
B. Ra
C. Fe
D. Al

Answer

To determine which of the given elements belongs to the category of transition metals, we need to understand what transition metals are and how to identify them based on their electron configurations.

### Step-by-Step Solution:

1. **Define Transition Metals**:
Transition metals are defined as elements that have an incomplete d subshell in their elemental form or in any of their oxidation states. They are typically found in the d-block of the periodic table.

2. **Identify the Elements**:
We need to examine the electron configurations of the given elements to see where their last electron enters. The elements mentioned in the video are potassium (K), radium (Ra), iron (Fe), and aluminum (Al).

3. **Check Electron Configuration**:
- **Potassium (K)**: The electron configuration is \( [Ar] 4s^1 \). The last electron enters the 4s subshell, which means potassium is an s-block element.
- **Radium (Ra)**: The electron configuration is \( [Rn] 7s^2 \). The last electron enters the 7s subshell, making radium an s-block element as well.
- **Iron (Fe)**: The electron configuration is \( [Ar] 4s^2 3d^6 \). The last electron enters the 3d subshell, which classifies iron as a transition metal (d-block element).
- **Aluminum (Al)**: The electron configuration is \( [Ne] 3s^2 3p^1 \). The last electron enters the 3p subshell, indicating that aluminum is a p-block element.

4. **Conclusion**:
Among the elements analyzed, only iron (Fe) has its last electron entering the d subshell, which qualifies it as a transition metal. Therefore, the correct answer is **iron (Fe)**.