The octet rule is not obeyed in :

Options

A. `CO_(2)`
B. `BCl_(3)`
C. `PCl_(5)`
D. (b) and (c) both

Answer

To determine which of the given compounds do not obey the octet rule, we will analyze each compound one by one. The octet rule states that atoms tend to gain, lose, or share electrons to have a total of eight electrons in their valence shell.

### Step-by-Step Solution:

1. **Understanding the Octet Rule**:
- The octet rule states that atoms are most stable when they have eight electrons in their outermost shell. This can be achieved through the loss, gain, or sharing of electrons.

2. **Analyzing CO2 (Carbon Dioxide)**:
- The structure of CO2 can be represented as O=C=O.
- Carbon (C) is the central atom, and it forms double bonds with two oxygen (O) atoms.
- Each double bond consists of 4 electrons (2 pairs), so:
- Carbon has 4 electrons from the two double bonds.
- Each oxygen has 4 electrons from the double bond with carbon.
- Total valence electrons around carbon = 4 (from bonds) + 4 (from one oxygen) + 4 (from the other oxygen) = 8.
- **Conclusion**: CO2 obeys the octet rule.

3. **Analyzing BCl3 (Boron Trichloride)**:
- The structure of BCl3 can be represented as Cl-B-Cl with one bond to each chlorine.
- Boron (B) is the central atom and forms three single bonds with three chlorine (Cl) atoms.
- Each bond consists of 2 electrons, so:
- Total valence electrons around boron = 3 (bonds) × 2 = 6.
- **Conclusion**: Boron has only 6 electrons in its valence shell, which means it does not obey the octet rule. Boron is considered hypovalent.

4. **Analyzing PCl5 (Phosphorus Pentachloride)**:
- The structure of PCl5 can be represented as Cl-Cl-Cl-P-Cl-Cl.
- Phosphorus (P) is the central atom and forms five single bonds with five chlorine (Cl) atoms.
- Each bond consists of 2 electrons, so:
- Total valence electrons around phosphorus = 5 (bonds) × 2 = 10.
- **Conclusion**: Phosphorus has 10 electrons in its valence shell, which means it does not obey the octet rule. Phosphorus is considered hypervalent.

5. **Final Conclusion**:
- From the analysis, we find that:
- CO2 obeys the octet rule.
- BCl3 does not obey the octet rule (hypovalent).
- PCl5 does not obey the octet rule (hypervalent).
- Therefore, the compounds that do not obey the octet rule are BCl3 and PCl5.

### Answer:
The octet rule is not obeyed in BCl3 and PCl5.