Question

Choose the correct option:
Assertion: $AlC{l}_3$ forms a dimer but $BC{l}_3$ does not form a dimer.
Reason: $AlC{l}_3$ does not exhibit back bonding because of large size of aluminum.

Answer 1

The halides of Boron i.e. $B{X}_3$ halides (where $X$ is halogen) attain an octet by $\pi$ back bonding but the other atoms of the same group have larger size and thus, we do not get effective $\pi$ back bonding in those cases, so they generally dimerise or polymerise to remedy the electron deficiency.
Hence $AlC{l}_3$ undergoes dimerisation to form $A{l}_{2}C{l}_6$.

Thus, we can say that Assertion and Reason both are correct and Reason is correct explanation of Assertion.