The number of d – electrons in `[Cr(H_2O)_6]^(3+)` [ATOMIC NO. OF Cr = 24] is

Options

A. 2
B. 3
C. 4.159 g
D. 5

Answer

To find the number of d-electrons in the complex ion \([Cr(H_2O)_6]^{3+}\), we will follow these steps:

### Step 1: Determine the oxidation state of chromium in the complex
In the complex \([Cr(H_2O)_6]^{3+}\), water (H₂O) is a neutral ligand, which means it does not contribute any charge. Since the overall charge of the complex is +3, the oxidation state of chromium must also be +3.

**Hint:** Remember that the charge of the complex is the sum of the oxidation states of the metal and the ligands.

### Step 2: Write the electron configuration of chromium
The atomic number of chromium (Cr) is 24. The electron configuration of neutral chromium is:
\[ Cr: [Ar] 4s^1 3d^5 \]
This means chromium has 1 electron in the 4s subshell and 5 electrons in the 3d subshell.

**Hint:** Use the periodic table to find the atomic number and write the electron configuration based on the order of filling.

### Step 3: Adjust the electron configuration for the oxidation state
Since we determined that chromium is in the +3 oxidation state, we need to remove electrons from its electron configuration. Electrons are removed first from the 4s subshell before the 3d subshell.

From the configuration:
- Remove 1 electron from the 4s subshell: \(4s^1\) becomes \(4s^0\).
- Remove 2 electrons from the 3d subshell: \(3d^5\) becomes \(3d^3\).

Thus, the electron configuration for \([Cr]^{3+}\) becomes:
\[ [Ar] 3d^3 \]

**Hint:** Remember that when determining the oxidation state, the order of electron removal is important: 4s electrons are removed before 3d electrons.

### Step 4: Count the d-electrons
From the adjusted electron configuration \([Ar] 3d^3\), we can see that there are 3 electrons in the 3d subshell.

**Hint:** Simply count the number of electrons in the d subshell after adjusting for the oxidation state.

### Conclusion
The number of d-electrons in \([Cr(H_2O)_6]^{3+}\) is **3**.

**Final Answer:** 3 d-electrons