Consult the table of standard electrode potentials

Substances that are stronger oxidising agents than ferrous ions can oxidise ferrous ions.

$\mathrm{Fe}^{2+} \longrightarrow \mathrm{Fe}^{3+}+\mathrm{e}^{-1} ; E^{\ominus}=-0.77 \mathrm{~V}$

This implies that the substances having higher reduction potentials than
+0.77 V can oxidise ferrous ions to ferric ions. Three substances that can do so are F2, Cl2, and O2.