Question

Halide of which element exist only in monomeric form is/are:

• A. boron
• B. aluminium
• C. both $A$ and $B$
• D. none of the above

Answer 1

The correct option is A boron

Option $\left(A\right)$ is correct.

Halides of boron exist only in monomeric form because boron atom is too small to coordinate with large halide ions.The energy released during the formation of the bridge structure is not sufficient for the cleavage of the typical $p\pi -p\pi$ bond in $B{X}_3$.

Other members of the group exist as dimmers having halogen - bridged structure.