Chromate ion, `CrO_4^(2-)` is

Options

A. Diamagnetic, orange in colour
B. Paramagnetic, yellow in colour
C. Diamagnetic, yellow in colour
D. Paramagnetic, orange in colour

Answer

To determine the properties of the chromate ion, \( \text{CrO}_4^{2-} \), we will follow these steps:

### Step 1: Determine the oxidation state of chromium in \( \text{CrO}_4^{2-} \)
Let the oxidation state of chromium be \( x \). The oxidation state of oxygen is typically \(-2\).

The overall charge of the chromate ion is \(-2\). Therefore, we can set up the equation:

\[ x + 4(-2) = -2 \]

This simplifies to:

\[ x - 8 = -2 \]

Now, solving for \( x \):

\[ x = -2 + 8 = +6 \]

### Step 2: Determine the electronic configuration of chromium in the +6 oxidation state
The electronic configuration of chromium (Cr) in its elemental form is:

\[ \text{Cr}: [\text{Ar}] 4s^1 3d^5 \]

When chromium is in the +6 oxidation state, it loses 6 electrons (5 from 3d and 1 from 4s):

\[ \text{Cr}^{6+}: [\text{Ar}] 4s^0 3d^0 \]

### Step 3: Analyze the presence of unpaired electrons
In the +6 oxidation state, all electrons have been removed, resulting in a completely filled configuration with no unpaired electrons. Therefore, \( \text{Cr}^{6+} \) has:

- No unpaired electrons
- It is diamagnetic.

### Step 4: Determine the color of the chromate ion
The chromate ion \( \text{CrO}_4^{2-} \) is known to have an intense yellow color. This color is not due to unpaired electrons but rather due to ligand-to-metal charge transfer (LMCT).

### Conclusion
Based on the analysis:
- The chromate ion \( \text{CrO}_4^{2-} \) is **diamagnetic** (due to no unpaired electrons).
- It is **yellow** in color (due to LMCT).

Thus, the correct answer is that the chromate ion \( \text{CrO}_4^{2-} \) is **diamagnetic and yellow in color**.

### Final Answer
The correct option is: **Diamagnetic yellow in color** (Option 3).

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