Can $Al_2O_3$ be converted into anhydrous $AlCl_3$ by heating? When?

In the nitrogen group,as we move from $N$ to $Sb$,the $H-M-H$ bond angle in the hydrides approaches $90^o$ because ......

Which of the following substances releases nitrogen gas upon thermal decomposition?

$N_2$ combines with metals to form:

During the borax bead test with $CuSO_4$,a blue-green colour of the bead was observed in the oxidising flame due to the formation of:

Account for the following observations:
$(A)$ $AlCl_3$ is a Lewis acid.
$(B)$ Though fluorine is more electronegative than chlorine yet $BF_3$ is a weaker Lewis acid than $BCl_3$.
$(C)$ $PbO_2$ is a stronger oxidizing agent than $SnO_2$.
$(D)$ The $+1$ oxidation state of thallium is more stable than its $+3$ state.