The empirical formula represents the simplest whole number ratio of atoms in a compound.

To find the empirical formula of N₃P₃Cl₆, we can see that the subscripts are already in the simplest whole number ratio:

* N:P:Cl = 3:3:6

Since the subscripts are already the smallest possible whole numbers, the empirical formula is the same as the molecular formula: N₃P₃Cl₆.