The number of molecules having lone pair of electrons on central atom in the following is

$\mathrm{BF}_3, \mathrm{SF}_4, \mathrm{SiCl}_4, \mathrm{XeF}_4, \mathrm{NCl}_3, \mathrm{XeF}_6, \mathrm{PCl}_5, \mathrm{HgCl}_2, \mathrm{SnCl}_2$

Observe the following molecules/ions $\mathrm{NH}_4^{+}, \mathrm{NH}_3, \mathrm{BF}_3, \mathrm{OH}^{-}, \mathrm{CH}_3^{+}, \mathrm{H}^{+}, \mathrm{CO}, \mathrm{C}_2 \mathrm{H}_4$.

The number of Lewis bases in the above list is

Identify the set containing isoelectronic species