Number of electron deficient molecules among the following
PH₃, B₂H₆, CCl₄, NH₃, LiH and BCl₃ is

Solution

Chemistry ma’am · 3 steps

Look for boron: with only 3 valence electrons, it falls short of an octet in both its compounds here.

1. What deficient means. A covalent molecule is electron deficient when its central atom ends up with fewer than 8 electrons, or when it has too few electrons for normal 2-electron bonds. Count the electrons around the central atom in each molecule.

   Species	Electrons around the central atom	Electron deficient?
   PH₃	3 bond pairs + 1 lone pair = 8 on P	No
   B₂H₆	12 valence electrons in total, too few for normal bonds	Yes
   CCl₄	4 bond pairs = 8 on C	No
   NH₃	3 bond pairs + 1 lone pair = 8 on N	No
   LiH	Ionic: Li⁺ and H⁻, each with a stable duplet	No
   BCl₃	3 bond pairs = only 6 on B	Yes

2. Why diborane. B₂H₆ has 2 × 3 + 6 × 1 = 12 valence electrons. A structure with 8 ordinary 2-electron bonds would need 16, so two hydrogens form 3-centre 2-electron bridge bonds instead.
3. So. Only B₂H₆ and BCl₃ are electron deficient, so the count is 2.

The idea to keep. Electron deficient means too few electrons to complete the central atom's octet or to form normal 2-electron bonds. Boron and aluminium compounds are the usual examples; ionic hydrides like LiH are not.

Answer from the NTA final answer key. Solution by PrepWiser.