JEE Main · 2022 · 25 Jun 2022 · Shift 1 · Chemical Bonding and Molecular Structure

Electron deficient species are those that have less than 8 electrons in their valence shell (incomplete octet), or less than 2 electrons for hydrogen.

Analyzing each molecule:

• ${\mathrm{PH}}_3$: P has 5 valence electrons + 3 from H = 8 electrons (complete octet)
• ${\mathrm{B}}_2{\mathrm{H}}_6$: Each B has only 6 electrons in valence shell (incomplete octet) — electron deficient
• ${\mathrm{CCl}}_4$: C has 4 + 4 = 8 electrons (complete octet)
• ${\mathrm{NH}}_3$: N has 5 + 3 = 8 electrons (complete octet)
• $\mathrm{LiH}$: Both Li and H have 2 electrons (complete for H)
• ${\mathrm{BCl}}_3$: B has 3 + 3 = 6 electrons (incomplete octet) — electron deficient

Therefore, 2 molecules (${\mathrm{B}}_2{\mathrm{H}}_6$ and ${\mathrm{BCl}}_3$) are electron deficient.