Consider the following oxoacids of phosphorus : $H_{3}P{O}_4$, $H_{3}P{O}_3$ and $H_{3}P{O}_2$ All ' P ' atoms in the oxoacids and oxoanions are 4 coordinate and contain at least one P=O unit. All ' P ' atoms in oxoacids have at least one P-OH group which ionises to give $H^{+}$ion. The oxidation state of ' P ' is +5 only when it is directly attached to 4 oxygen atoms. For every two units decrease in oxidation state of Phosphorus, there is one P-H bond.

The number of H-atoms directly attached to 'P' in one molecule of each of ${\mathrm{H}}_3{\mathrm{PO}}_4,{\mathrm{H}}_3{\mathrm{PO}}_3\text{ and }{\mathrm{H}}_3{\mathrm{PO}}_2$are respectively