, ▁and ▁their ▁est ers ▁and ▁sal ts ▁(" ph osph ates ") ▁include ▁some ▁of ▁the ▁best - known ▁and ▁most ▁important ▁comp ounds ▁of ▁ph osph orus . ▁▁▁▁ ▁The ▁simplest ▁member ▁of ▁this ▁class ▁is ▁ ▁Ph osph or ic ▁acid ▁proper , ▁also ▁called ▁orth oph osph or ic ▁or ▁mon oph osph or ic ▁acid , ▁ ▁( or ▁), ▁a ▁tri pro t ic ▁acid . ▁It ▁forms ▁orth oph osph ate ▁salt ▁and ▁est ers , ▁commonly ▁called ▁ph osph ates . ▁ ▁The ▁smallest ▁comp ounds ▁of ▁this ▁class ▁with ▁two ▁or ▁more ▁ph osph orus ▁atoms ▁are ▁called ▁" ol ig oph osph or ic ▁ac ids ", ▁and ▁the ▁larger ▁ones , ▁with ▁linear ▁– P – O – ▁back b ones , ▁are ▁" poly ph osph or ic ▁ac ids "; ▁with ▁no ▁definite ▁separation ▁between ▁the ▁two . ▁Some ▁of ▁the ▁most ▁important ▁members ▁are : ▁▁ ▁Py roph osph or ic ▁acid , ▁ ▁( or ▁– O – ), ▁with ▁four ▁acid ▁hyd rog ens . ▁ ▁Form s ▁py roph osph ates . ▁ ▁Tri ph osph or ic ▁or ▁tri poly ph osph or ic ▁acid , ▁ ▁( or ▁– O – – O – ), ▁with ▁five ▁acid ic ▁hyd rog ens . ▁ ▁Form s ▁tri ph osph ates ▁or ▁tri poly ph osph ates . ▁ ▁T et raph osph or ic ▁acid , ▁ ▁( or ▁( – O – )