A compound is a substance made up of atoms of different elements. The number of atoms each element posseses make up the chemical formula of the compound. In this question, a certain compound is made up of three chlorine atoms (Cl3), one phosphorus atom (P), and one oxygen atom (O).
Combining these atoms of different quantity together, this means that the compound will have a chemical formula: POCl3. This compound is called phosphorus oxychloride.
