Which of the following molecule does NOT obey octet rule?

Options

A. `BF_(3)`
B. `CO_(2)`
C. `H_(2)O`
D. `N_(2)`

Answer

To determine which molecule does not obey the octet rule, we will analyze the valence electrons of each molecule's central atom after bonding. The octet rule states that atoms tend to bond in such a way that they have eight electrons in their valence shell.

### Step-by-Step Solution:

1. **Identify the Molecules**: The given molecules are BF3, CO2, H2O, and N2.

2. **Analyze BF3**:
- **Central Atom**: Boron (B)
- **Valence Electrons in Boron**: 3
- **Bonding with Fluorine (F)**: Each fluorine has 7 valence electrons.
- **Lewis Structure**: Boron forms three single bonds with three fluorine atoms.
- **Total Electrons in Valence Shell of Boron After Bonding**: 6 (3 from bonds with F).
- **Conclusion**: Boron has less than 8 electrons. **BF3 is hypovalent and does not obey the octet rule.**

3. **Analyze CO2**:
- **Central Atom**: Carbon (C)
- **Valence Electrons in Carbon**: 4
- **Bonding with Oxygen (O)**: Each oxygen has 6 valence electrons.
- **Lewis Structure**: Carbon forms double bonds with two oxygen atoms.
- **Total Electrons in Valence Shell of Carbon After Bonding**: 8 (4 from bonds).
- **Conclusion**: Carbon has exactly 8 electrons. **CO2 obeys the octet rule.**

4. **Analyze H2O**:
- **Central Atom**: Oxygen (O)
- **Valence Electrons in Oxygen**: 6
- **Bonding with Hydrogen (H)**: Each hydrogen has 1 valence electron.
- **Lewis Structure**: Oxygen forms two single bonds with two hydrogen atoms.
- **Total Electrons in Valence Shell of Oxygen After Bonding**: 8 (6 from O + 2 from H).
- **Conclusion**: Oxygen has exactly 8 electrons. **H2O obeys the octet rule.**

5. **Analyze N2**:
- **Central Atom**: Nitrogen (N)
- **Valence Electrons in Nitrogen**: 5
- **Bonding**: Each nitrogen forms a triple bond with another nitrogen.
- **Total Electrons in Valence Shell of Nitrogen After Bonding**: 8 (6 from the triple bond).
- **Conclusion**: Each nitrogen has exactly 8 electrons. **N2 obeys the octet rule.**

### Final Conclusion:
The molecule that does not obey the octet rule is **BF3**.

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