Number of electron deficient molecules among the following PH 3 , B 2 H 6 , CCl 4 , NH 3 , LiH and BCl 3 is

A molecule is said to be electron deficient because there are not enough valence electrons to form the expected number of covalent bonds. For example, in ${\mathrm{B}}_2{\mathrm{H}}_6$, there are only 12 electrons, three from each boron atoms and six from hydrogen, while ethane possesses 14 such electrons.

Also in ${\mathrm{BCl}}_3$, B has six electrons in its outermost orbit. So, the number of electron deficient molecules are 2.