In which of the following compounds of Cr, the oxidation number Cr is not `+6` :-

Options

A. `CrO_(3)`
B. `CrO_(2)Cl_(2)`
C. `Cr_(2)O_(3)`
D. `K_(2)Cr_(2)O_(7)`

Answer

To determine the oxidation number of chromium (Cr) in the given compounds and identify which one does not have an oxidation state of +6, we will analyze each compound step by step.

### Step 1: Analyze CrO3
- **Compound**: CrO3
- **Oxidation states**:
- Let the oxidation number of Cr be \( X \).
- Oxygen (O) has an oxidation number of -2.
- **Equation**:
\[ X + 3(-2) = 0 \]
\[ X - 6 = 0 \implies X = +6 \]
- **Conclusion**: The oxidation number of Cr in CrO3 is +6.

### Step 2: Analyze CrO2Cl2
- **Compound**: CrO2Cl2
- **Oxidation states**:
- Let the oxidation number of Cr be \( X \).
- Oxygen (O) has an oxidation number of -2, and chlorine (Cl) has an oxidation number of -1.
- **Equation**:
\[ X + 2(-2) + 2(-1) = 0 \]
\[ X - 4 - 2 = 0 \implies X - 6 = 0 \implies X = +6 \]
- **Conclusion**: The oxidation number of Cr in CrO2Cl2 is +6.

### Step 3: Analyze Cr2O3
- **Compound**: Cr2O3
- **Oxidation states**:
- Let the oxidation number of Cr be \( X \).
- **Equation**:
\[ 2X + 3(-2) = 0 \]
\[ 2X - 6 = 0 \implies 2X = 6 \implies X = +3 \]
- **Conclusion**: The oxidation number of Cr in Cr2O3 is +3.

### Step 4: Analyze K2Cr2O7
- **Compound**: K2Cr2O7
- **Oxidation states**:
- Let the oxidation number of Cr be \( X \).
- Potassium (K) has an oxidation number of +1.
- **Equation**:
\[ 2(+1) + 2X + 7(-2) = 0 \]
\[ 2 + 2X - 14 = 0 \implies 2X - 12 = 0 \implies 2X = 12 \implies X = +6 \]
- **Conclusion**: The oxidation number of Cr in K2Cr2O7 is +6.

### Final Conclusion:
Among the given compounds, the oxidation number of chromium is not +6 in **Cr2O3**, where it is +3.

### Answer:
**Cr2O3** (Option 3) is the compound where the oxidation number of Cr is not +6.

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