A `M^(2+)` ion derived from a metal in the first transition metal series has four electrons in `3d` subshell. What element might `M` be?

Explanation

To determine which metal \( M \) corresponds to a \( M^{2+} \) ion with four electrons in the \( 3d \) subshell, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Electron Configuration**:
The \( M^{2+} \) ion has four electrons in the \( 3d \) subshell. This means that in its neutral state, the metal \( M \) must have a total of six electrons in the \( 3d \) subshell (since two electrons are lost to form the \( M^{2+} \) ion).

2. **Determine the Total Number of Electrons**:
The electron configuration for transition metals in the first series (from Scandium to Zinc) can be expressed as:
\[ \text{[Ar]} \, 4s^2 \, 3d^n \]
where \( n \) is the number of electrons in the \( 3d \) subshell. Since we need \( n = 6 \) for the neutral atom, the configuration becomes:
\[ \text{[Ar]} \, 4s^2 \, 3d^6 \]

3. **Identify the Element**:
The atomic number of the element can be calculated by adding the number of electrons in the \( 4s \) and \( 3d \) subshells:
\[ 18 \, (\text{from } \text{[Ar]}) + 2 \, (4s^2) + 6 \, (3d^6) = 26 \]
Thus, the atomic number of the element \( M \) is 26.

4. **Find the Element**:
The element with atomic number 26 is Iron (Fe).

5. **Conclusion**:
Therefore, the metal \( M \) is Iron (Fe).

### Final Answer:
The element \( M \) is Iron (Fe).

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