In which of the following molecules does the central atom not follow the octet rule?

Options

A. `CO_(2)`
B. `H_(2)O`
C. `O_(2)`
D. `NO`

Explanation

To determine which of the given molecules has a central atom that does not follow the octet rule, we will analyze each molecule's Lewis structure step by step.

### Step 1: Analyze Carbon Dioxide (CO₂)
1. **Identify the central atom**: Carbon (C) is the central atom.
2. **Count valence electrons**: Carbon has 4 valence electrons, and each oxygen (O) has 6, giving a total of 16 valence electrons for CO₂ (4 + 2×6 = 16).
3. **Draw the Lewis structure**: Carbon forms double bonds with each oxygen atom:
- O=C=O
4. **Check the octet**: Carbon shares 4 electrons (2 from each double bond) and has a total of 8 electrons around it.
5. **Conclusion**: Carbon in CO₂ follows the octet rule.

### Step 2: Analyze Water (H₂O)
1. **Identify the central atom**: Oxygen (O) is the central atom.
2. **Count valence electrons**: Oxygen has 6 valence electrons, and each hydrogen (H) has 1, giving a total of 8 valence electrons for H₂O (6 + 2×1 = 8).
3. **Draw the Lewis structure**: Oxygen forms single bonds with two hydrogen atoms:
- H-O-H
4. **Check the octet**: Oxygen has 6 of its own electrons plus 2 shared from hydrogen, totaling 8 electrons.
5. **Conclusion**: Oxygen in H₂O follows the octet rule.

### Step 3: Analyze Oxygen (O₂)
1. **Identify the central atom**: Both oxygen atoms are equivalent.
2. **Count valence electrons**: Each oxygen has 6 valence electrons, giving a total of 12 for O₂ (6 + 6 = 12).
3. **Draw the Lewis structure**: The two oxygen atoms form a double bond:
- O=O
4. **Check the octet**: Each oxygen atom shares 4 electrons (2 from the double bond), totaling 8 electrons around each.
5. **Conclusion**: Both oxygen atoms in O₂ follow the octet rule.

### Step 4: Analyze Nitric Oxide (NO)
1. **Identify the central atom**: Nitrogen (N) is the central atom.
2. **Count valence electrons**: Nitrogen has 5 valence electrons, and oxygen has 6, giving a total of 11 valence electrons for NO (5 + 6 = 11).
3. **Draw the Lewis structure**: Nitrogen forms a double bond with oxygen:
- N=O
4. **Check the octet**: Nitrogen has 5 electrons (from its own) and shares 2 with oxygen, totaling 7 electrons around it.
5. **Conclusion**: Nitrogen in NO does not complete its octet.

### Final Conclusion
The molecule in which the central atom does not follow the octet rule is **Nitric Oxide (NO)**.

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