Question 73: Amongst the following, the total number of species NOT having eight electrons around central atom in its outer most shell, is \mathrm{NH}_{3}, \mathrm{AlCl}_{3}, \mathrm{BeCl}_{2}, \mathrm{CCl}_{4}, \mathrm{PCl}_{5} :
(1) 4
(2) 1
(3) 3
(4) 2
Answer: Option (3)
Explanation:
According to the octet rule, a central atom is stable when it has eight electrons in its valence shell.
In \mathrm{NH}_{3}, nitrogen has three bond pairs and one lone pair,
giving a total of eight electrons around nitrogen, so it follows the octet rule.
In \mathrm{AlCl}_{3}, aluminium forms three bonds with chlorine and has
only six electrons around it, so it does not follow the octet rule.
In \mathrm{BeCl}_{2}, beryllium forms two bonds and has only four electrons around it,
so it does not follow the octet rule.
In \mathrm{CCl}_{4}, carbon forms four covalent bonds and has eight electrons around it, so it follows the octet rule.
In \mathrm{PCl}_{5}, phosphorus has five bond pairs, giving ten electrons around it,
so it does not follow the octet rule.
Thus, the species not having eight electrons around the central atom are \mathrm{AlCl}_{3},
\mathrm{BeCl}_{2}, and \mathrm{PCl}_{5}.
The total number of such species is 3, hence the correct answer is option (3).