How many 'd' electrons are present in `Cr^(2+)` ion?

Options

A. 4
B. 5
C. 6
D. 3

Answer

To determine the number of 'd' electrons present in the `Cr^(2+)` ion, we will follow these steps:

### Step 1: Determine the Atomic Number of Chromium
Chromium (Cr) has an atomic number of 24. This means it has 24 electrons in its neutral state.

### Step 2: Write the Expected Electronic Configuration of Chromium
The expected electronic configuration of chromium based on the Aufbau principle is:
\[ \text{Cr: } 1s^2, 2s^2, 2p^6, 3s^2, 3p^6, 4s^2, 3d^4 \]

### Step 3: Identify the Actual Electronic Configuration of Chromium
However, chromium has a unique configuration due to stability reasons. The actual electronic configuration is:
\[ \text{Cr: } 1s^2, 2s^2, 2p^6, 3s^2, 3p^6, 4s^1, 3d^5 \]
This configuration is more stable because having half-filled 'd' orbitals (3d^5) is energetically favorable.

### Step 4: Determine the Configuration of Chromium in the `Cr^(2+)` Ion
When chromium loses two electrons to form the `Cr^(2+)` ion, the electrons are removed first from the 4s orbital and then from the 3d orbital. Therefore, the electronic configuration for `Cr^(2+)` is:
\[ \text{Cr}^{2+}: 1s^2, 2s^2, 2p^6, 3s^2, 3p^6, 4s^0, 3d^4 \]

### Step 5: Count the 'd' Electrons in `Cr^(2+)`
From the electronic configuration of `Cr^(2+)`, we can see that there are 4 electrons in the 3d orbital.

### Conclusion
Thus, the number of 'd' electrons present in the `Cr^(2+)` ion is **4**.

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