Arsenic can form ions with different charges. Here are the most common ones:

* Arsenite ion: AsO₃⁻³

* Arsenate ion: AsO₄⁻³

The symbol depends on the oxidation state of arsenic.

* Arsenite ion (AsO₃⁻³) has arsenic in the +3 oxidation state.

* Arsenate ion (AsO₄⁻³) has arsenic in the +5 oxidation state.

So, to know the exact symbol, you need to know the oxidation state of the arsenic ion in question.