Which of the following has a different number of $p_{\pi}-d_{\pi}$ bonds compared to the other molecules?

The structure of $B_3N_3H_6$ (borazine) is as follows. How many derivative structures of $B_3N_3H_4X_2$ can be derived from the basic structure by the replacement of two hydrogen atoms?

Which of the following cations is the most stable?

In the solid state,$PCl_5$ exists as which of the following?

For the following molecules/ions,identify whether the octet rule is obeyed or not obeyed. Provide the reason.
$(1)$ $PCl_5$ $(2)$ $SF_6$ $(3)$ $PF_5$ $(4)$ $NH_4^+$ $(5)$ $BeCl_2$ $(6)$ $NCl_3$

The $P-P-P$ bond angle in $P_4$ and $S-S-S$ bond angle in cyclo $S_8$ molecule are respectively