Structures of BeCl$$_2$$ in solid state, vapour phase and at very high temperature respectively are:

We need to determine the structures of $$\text{BeCl}_2$$ in the solid state, vapour phase, and at very high temperature.

In the solid state, $$\text{BeCl}_2$$ exists as a polymeric chain structure. Each Be atom is tetrahedrally coordinated, bonded to four Cl atoms through bridging chlorine atoms forming a chain polymer.

When $$\text{BeCl}_2$$ is vaporised at moderate temperatures, the polymeric chain breaks down into dimeric units. In the dimer, two BeCl$$_2$$ units are connected through two bridging Cl atoms, with each Be atom having a coordination number of 3 (trigonal planar geometry with two terminal and one bridging Cl).

At very high temperatures, the dimer dissociates completely into monomeric BeCl$$_2$$ molecules. The monomer is a linear molecule (sp hybridisation) with Be bonded to two Cl atoms at 180°.

Solid state → Polymeric, Vapour phase → Dimeric, Very high temperature → Monomeric. The correct answer is Option D: Polymeric, Dimeric, Monomeric.