Which of the following structures does not represent the structure of the substance given in the brackets?

Which of the following statements about $BF_4^{-}$ and $AlF_6^{3-}$ are correct?
$(i)$ $B$ and $Al$ differ in their oxidation states.
$(ii)$ $B$ and $Al$ differ in their covalency.
$(iii)$ $B$ obeys the octet rule.
$(iv)$ $B$ and $Al$ are in diagonal relationship.

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

The hardest substance amongst the following is

Which of the following statements is not correct about diborane $(B_2H_6)$?

When orthoboric acid $(H_3BO_3)$ is subjected to strong heating, the residue left is