Structures of BeCl₂ in solid state, vapour phase and at very high temperature respectively are :

Solution

Chemistry ma’am · 3 steps

Heat breaks the Cl bridges one stage at a time: chain, then pair, then single linear molecule.

1. Why bridges form. Be in BeCl₂ has only 4 electrons around it, so it is electron deficient. A lone pair on the Cl of a neighbouring molecule can fill this gap and form a Be-Cl-Be bridge.

   Condition	Structure	Hybridisation of Be	Shape around Be
   Solid state	Polymeric chain, each Be bonded to 4 bridging Cl	sp³	tetrahedral
   Vapour phase	Dimeric Be₂Cl₄, two bridging Cl	sp²	trigonal planar
   Very high temperature (about 1200 K)	Monomeric BeCl₂	sp	linear

2. Trend with heat. Raising the temperature breaks more and more bridges. The long chain becomes a dimer, and the dimer splits into separate linear molecules.
3. So. The order is polymeric, dimeric, monomeric, which is option (a).

The idea to keep. Electron-deficient halides like BeCl₂ and AlCl₃ use halogen bridges to make up the shortfall. The hotter the sample, the fewer the bridges and the smaller the unit.

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