Q. Halides of Boron and Aluminium?

All the elements of Group 13 form binary halides. All the four trihalides of each element are known, with one exception. The compound Tl13 is not thallium (III) iodide, but rather thallium (1) trioxide Tl⁺(l₃,)-, which is similar to potassium  triodide, K⁺I₃^-. You will learn more about triodides in Unit 10. Thallium (III) chloride and bromide are also very unstable and decompose into the thallium (1) halides and the free halogen. Thus, the only stable travailed of thallium in +3 oxidation states is the trifluoride, TIF3, which is an ionic solid. This is in keeping with the general trend that Tl(1) compounds are more stable than TI(I1I) compounds. Let us no briefly discuss the halides of B and Al.