The simplest formula for the compound formed between phosphorus and excess chlorine is PCl₅, which is phosphorus pentachloride.

Here's why:

* Phosphorus (P) is in group 15 of the periodic table and has a valency of 5 (meaning it can form 5 bonds).

* Chlorine (Cl) is in group 17 and has a valency of 1.

* In an excess of chlorine, phosphorus will form as many bonds as possible, leading to the formation of PCl₅.

Keep in mind that this compound is actually quite complex, and it exists in equilibrium with other forms, but PCl₅ is the simplest representation of its composition.