JEE Main · 2023 · 08 Apr 2023 · Shift 2 · Chemical Bonding and Molecular Structure

Pyrophosphoric acid, ${\mathrm{H}}_4{\mathrm{P}}_2{\mathrm{O}}_7$, has two phosphorus atoms joined through a bridging oxygen, with each phosphorus also carrying one terminal doubly bonded oxygen and two hydroxyl groups. Each phosphorus forms four phosphorus\u2013oxygen sigma bonds, to the terminal oxygen, to the bridging oxygen and to the two hydroxyl oxygens, giving eight such sigma bonds in all, and the four hydroxyl groups add four oxygen\u2013hydrogen sigma bonds, making twelve sigma bonds. The two terminal phosphorus\u2013oxygen double bonds provide the only two pi bonds. The ratio of sigma to pi bonds is twelve to two, that is 6.