JEE Main 2023 · 11 April · evening · Single correct · +4 / −1
Which hydride among the following is less stable?
- (a) NH₃
- (b) HF
- (c) LiH
- (d) BeH₂
Answer and solution
Answer (d) BeH₂
Solution
Chemistry ma’am · 3 steps
Check whether the central atom gets a full octet. Beryllium in BeH₂ is left with only 4 electrons.
- What decides stability. A hydride is stable when the bonds to hydrogen are strong and the central atom is comfortable, either as an ion with a full shell or with a complete octet. A hydride that is short of electrons tends to break down easily.
Hydride Type Electrons around the central atom Stability NH₃ molecular (covalent) 8, full octet stable HF molecular (covalent) 8 on F, very strong H–F bond very stable LiH ionic, Li⁺ H⁻ both ions have a noble-gas shell stable, melts without decomposing BeH₂ covalent, polymeric only 4 on Be, electron deficient least stable - Why BeH2 is weak. Beryllium is very small and forms covalent bonds, so it does not form an ionic hydride like Li does. With only two Be–H bonds it is electron deficient, so it links into chains through three-centre two-electron bridges. It cannot be made directly from Be and H₂, and it decomposes on mild heating.
- So. BeH₂ is the least stable of the four, option (d).
The idea to keep. For hydride stability, look for electron deficiency. A central atom left with fewer than 8 electrons, like Be in BeH₂ or B in boranes, gives a weaker, easily decomposed hydride.
Answer from the NTA final answer key. Solution by PrepWiser.