In which of the following molecules is a hydrogen bridge bond present?

The correct order of atomic radii in group $13$ elements is

Name the metals with which $Al$ can form alloys.

$A$ nonmetallic element of group $13$,used in making bulletproof vests,is an extremely hard solid of black colour. It can exist in many allotropic forms and has an unusually high melting point. Its trifluoride acts as a Lewis acid towards ammonia. The element exhibits a maximum covalence of four. Identify the element and write the reaction of its trifluoride with ammonia. Explain why the trifluoride acts as a Lewis acid.

Give the physical and chemical properties of orthoboric acid $(H_3BO_3)$.

The incorrect stability order of $+3$ and $+1$ states of $13^{th}$ group elements (boron family) is