Distinguish between the Lewis structures of the following pairs of molecules:
$(i)$ $BCl_3$ and $BH_3$
$(ii)$ $BeCl_2$ and $BeH_2$

(N/A) In $BCl_3$,the $B$ atom has an incomplete octet (sextet),while each $Cl$ atom has a complete octet.
In $BH_3$,the $B$ atom has an incomplete octet (sextet),and the $H$ atoms have a duplet configuration (not an octet).
In $BeCl_2$,the $Be$ atom has an incomplete octet (quartet),while each $Cl$ atom has a complete octet.
In $BeH_2$,the $Be$ atom has an incomplete octet (quartet),and the $H$ atoms have a duplet configuration.
Thus,in both pairs,the central atom ($B$ or $Be$) has an incomplete octet,but the terminal atoms differ in their valence shell completion ($Cl$ completes its octet,whereas $H$ completes its duplet).