In Which Pair Of Species, The Octet Rule Is Not Obeyed?

The correct option is C

${\mathrm{BCl}}_3\mathrm{And}{\mathrm{PCl}}_5$

Explanation for the correct option:

Option(C): ${\mathrm{BCl}}_3\mathrm{And}{\mathrm{PCl}}_5$

• An atom having electrons equal to 8 in its valence shell is known to obey the octet rule.
• Finding the species which does not obey the octet rule can be done by calculating the total number of electrons in the species which is given by the number of bonded electrons + the number of electrons in lone pairs for the central metal atom.
• In ${\mathrm{BCl}}_3$ , it has three bond pairs, no lone pair which it has a total of 6 electrons.
• Also in ${\mathrm{PCl}}_5$​ there are five bond pairs, no lone pair which means it has a total of 10 electrons.
• Hence this pair of species does not obey the octet rule.

Explanation for the incorrect option:

Option(A): ${\mathrm{CF}}_4\mathrm{And}{\mathrm{NF}}_3$

• In ${\mathrm{CF}}_4$ there are 4 bonding and 0 lone pair involved, which makes 8 electrons for obeying the octet rule.
• Similarly in${\mathrm{NF}}_3$ there are 3 bonding pairs and 1 lone pair makes 8 electrons in total.
• Hence this pair of species obey the octet rule.

Option(B): ${\mathrm{NH}}_3\mathrm{And}{\mathrm{NCl}}_3$

• In the case of both ${\mathrm{NH}}_3\mathrm{And}{\mathrm{NCl}}_3$, there are 3 bonding pairs and 1 lone pair makes 8 electrons in total.
• Thus this pair of species obey the octet rule.

Option(D): ${\mathrm{PF}}_3\mathrm{And}{\mathrm{POCl}}_3$

• In ${\mathrm{POCl}}_3$ there are 4 bonding and 0 lone pair involved, which makes 8 electrons for obeying the octet rule.
• Similarly in ${\mathrm{PF}}_3$ there are 3 bonding pairs and 1 lone pair makes 8 electrons in total.
• Hence this pair of species obey the octet rule.

Therefore, the Pair Of Species, in which the Octet Rule Is Not Obeyed is ${\mathrm{BCl}}_3\mathrm{And}{\mathrm{PCl}}_5$.