Observe the following molecules/ions: $NH_4^{+}, NH_3, BF_3, OH^{-}, CH_3^{+}, H^{+}, CO, C_2H_4$. The number of Lewis bases in the above list is:

The correct order of $C-O$ bond length among $CO, CO_3^{2-}, CO_2$ is

Select the correct order:
$(a)$ $CO < CO_2 < CO_3^{2-}$ $\Rightarrow$ Bond length
$(b)$ $O_2 < O_3 < O_2^{2-}$ $\Rightarrow$ Bond length
$(c)$ $N_2 < N_2^+$ $\Rightarrow$ Bond energy

$P_4O_{10}$ has short and long $P-O$ bonds. The number of short $(P=O)$ bonds in this compound is:

Which of the following cannot act as a Lewis base?

The isoelectronic pair is