Exceptions to the Octet Rule

The octet rule states that atoms tend to gain, lose, or share electrons in order to achieve a stable configuration of eight valence electrons. However, there are some molecules that do not follow this rule due to their unique electronic configurations. These molecules are called exceptions to the octet rule.

Molecules that Follow the Octet Rule

Before identifying the molecule that is not an exception to the octet rule, it's important to understand which molecules follow the rule. Generally, molecules that have atoms with four or fewer valence electrons tend to follow the octet rule. Some examples include:

- Methane (CH4)
- Carbon dioxide (CO2)
- Water (H2O)
- Ammonia (NH3)

Exceptions to the Octet Rule

Molecules that have atoms with more than four valence electrons tend to be exceptions to the octet rule. This is because these atoms can accommodate more than eight electrons due to their d-orbitals. Some examples include:

- Boron trifluoride (BF3)
- Phosphorus pentafluoride (PF5)
- Sulfur hexafluoride (SF6)
- Xenon tetrafluoride (XeF4)

Identifying the Molecule that is Not an Exception to the Octet Rule

Out of the given options, the molecule that is not an exception to the octet rule is carbon dioxide (CO2). This is because carbon and oxygen both have four valence electrons and are able to follow the octet rule by sharing electrons with each other. The Lewis structure of CO2 shows that each atom has a full octet of electrons.

Conclusion

In summary, the octet rule is a fundamental concept in chemistry that explains the stability of atoms and molecules. While most molecules tend to follow this rule, some exceptions exist due to unique electronic configurations. Out of the given options, carbon dioxide is not an exception to the octet rule as both carbon and oxygen follow the rule by sharing electrons with each other.

B. Co2 because bf3 - is a hypovalent means having less than 8 e- in valence shellwhile If7 and PF5 are hypervalent ie - having more than 8 e- in valence shell but CO2 is having exactly 8e- in valence shell