A certain compound is made up of one phosphorus (P) atom, three chlorine (Cl) atoms, and one oxygen (O) atom. What is the chemical formula of this compound?

It is PCl³O. The octet rule sais that atoms tend to gain, lose or share electrons to have 8 electrons in their outer electron shell. But some elements like P and S are exeptions. And this molecule has 3 P-Cl bonds and a P-O bond.