* Fluorine (F) is always assigned an oxidation number of -1 in its compounds, except in elemental fluorine (F₂).

* The sum of the oxidation numbers in a neutral molecule must equal zero.

Let's apply these rules to PF₃:

* We have three fluorine atoms, each with an oxidation number of -1, giving a total of -3.

* To make the molecule neutral, the oxidation number of phosphorus (P) must be +3.

Therefore, the oxidation number of phosphorus (P) in PF₃ is +3.