The formula for compound triphosphorus heptachloride is P₃Cl₇.

Here's how it breaks down:

* Triphosphorus: Indicates three phosphorus atoms (P₃).

* Heptachloride: Indicates seven chlorine atoms (Cl₇).

Therefore, the compound is composed of three phosphorus atoms and seven chlorine atoms, resulting in the formula P₃Cl₇.