JEE Main · 2021 · 18 Mar 2021 · Shift 2 · Chemical Bonding and Molecular Structure

The lone pairs left on a central atom follow from its valence electrons once the bonding pairs are taken out. Sulphur in $\mathrm{S}{\mathrm{F}}_4$ keeps one pair, arsenic in $\mathrm{As}{\mathrm{F}}_3$ one and bromine in $\mathrm{Br}{\mathrm{F}}_5$ one; boron in $\mathrm{B}{\mathrm{F}}_4^{-}$, phosphorus in $\mathrm{PC}{\mathrm{l}}_5$ and sulphur in $\mathrm{S}{\mathrm{F}}_6$ keep none. Chlorine in $\mathrm{Cl}{\mathrm{F}}_3$ uses three of its seven valence electrons in bonds and retains four electrons, that is two lone pairs, and xenon in $\mathrm{Xe}{\mathrm{F}}_4$ uses four of its eight and also retains two lone pairs. The number of species carrying two lone pairs on the central atom is $2$.