The structures of beryllium chloride in solid state and vapour phase, are :

The solid‑state structure of beryllium chloride (BeCl₂) is not molecular but polymeric. Each Be atom is electron‑deficient (only two valence electrons) and prefers to achieve a tetrahedral coordination by sharing its two chloride ligands with neighboring beryllium atoms. This results in a one‑dimensional infinite chain where each Be is surrounded by four Cl atoms and each Cl bridges two Be atoms.

In contrast, when BeCl₂ is vaporised the temperature is high enough that the extended polymer breaks down. The most stable gaseous species is a discrete dimer, Be₂Cl₄. In this dimer two Be atoms are linked by two bridging chloride ions, while each Be still holds two terminal chloride ligands. The overall geometry of the dimer can be described as two BeCl₂ units joined through shared Cl atoms, forming a compact molecular entity.

Thus, the structures are:

• Solid state – an infinite chain (polymer) of BeCl₂ units.
• Vapour phase – a molecular dimer, Be₂Cl₄.

This reasoning matches the option “Chain and dimer, respectively”.