Question

Observe the following reactions: \[ \text{Dimer} \xrightarrow{\text{vapour phase}} MCl_2 \xrightarrow{1200\,K} \text{linear monomer} \] What is \(M\)?

• A. Ca
• B. Sr
• C. Mg
• D. Be

Hint:

Beryllium compounds often show anomalous behavior because of the small size and high charge density of \(Be^{2+}\). In the vapour phase, \(BeCl_2\) exists as a bridged dimer \((Be_2Cl_4)\) and at high temperatures forms a linear monomer \((Cl-Be-Cl)\).

Answer 1

The Correct Option is D

Step 1: Recall the nature of Group 2 metal chlorides.
Among the alkaline earth metal chlorides, \(BeCl_2\) shows anomalous behavior due to the very small size and high polarizing power of the \(Be^{2+}\) ion.
As a result, \(BeCl_2\) possesses significant covalent character.

Step 2: Structure of \(BeCl_2\) in the vapour phase.
At ordinary temperatures in the vapour phase, \(BeCl_2\) exists as a dimer: \[ Be_2Cl_4 \] The dimer contains chlorine bridges between the two beryllium atoms.

Step 3: Effect of high temperature.
On heating to about \[ 1200\,K \] the bridged dimer dissociates into monomeric \(BeCl_2\).
The monomer has a linear structure because the central Be atom is \(sp\)-hybridized. \[ Cl-Be-Cl \] with bond angle \[ 180^\circ \]

Step 4: Examine the remaining options.
\(MgCl_2\), \(CaCl_2\), and \(SrCl_2\) are predominantly ionic compounds and do not show the characteristic dimer-to-linear monomer behavior observed for \(BeCl_2\).
Therefore, these options are incorrect.

Step 5: Final conclusion.
The compound undergoing the transformation \[ Be_2Cl_4 \rightarrow 2BeCl_2 \] to form a linear monomer at high temperature is beryllium chloride.
Hence, \[ \boxed{\text{Be}} \] Therefore, option (4) is correct.