Question

Give reason: $P$ in $PC{l_5}$ and $S$ in $S{F_6}$ having $10$ electrons and $12$ electrons in their valence shell.

Answer 1

It will be better to write the electronic configurations of the individual element phosphorus $P$ and Sulphur $S$. Then try to draw the structures of $PC{l_5}$ and $S{F_6}$ using the basics of chemical bonding.

Complete answer:
First, we will write the electronic configuration of the individual element $P$ and Sulphur $S$ . So the electronic configuration for the two elements can be written as follows:
The total number of electrons present in Phosphorus is $15$. So, the electronic configuration can be written as.
$P \to 1{s^2}2{s^2}2{p^6}3{s^2}3{p^3}3{d^0}$
The total number of electrons present in Sulphur is $16$. So, the electronic configuration can be written as.
$S \to 1{s^2}2{s^2}2{p^6}3{s^2}3{p^4}3{d^0}$
The molecule $PC{l_5}$ is formed by sharing one electron each with five chlorine atoms. We know that there are five electrons in the valence shell of phosphorus. But in phosphorus pentachloride $PC{l_5}$ chlorine has eight electrons with a complete octet. But $P$ now have $10$ electrons. Five electrons of phosphorus with the addition of five shared electrons with chlorine.
Now we will consider $S{F_6}$ which Sulphur with six electrons in the valence shell.
The molecule $S{F_6}$ is formed by sharing one electron each with six fluorine atoms.We know that there are six electrons in the valence shell of Sulphur. But in $S{F_6}$ fluorine have eight electrons with a complete octet. But $S$ have $12$ electrons. Six electrons of Sulphur with the addition of six shared electrons with fluorine.

Note:
In several compounds of the third period and beyond the third period there are more than eight valence electrons around the central atom. This condition is the limitation of octet rule also known as an expanded octet.
According to the octet rule, atoms can combine by sharing or transfer of electrons from one atom to another to complete their octet simply means eight electrons around the central atom.