Iron has variable valencies of Fe2+ and Fe3+
Being in the transition area, Iron has an incomplete subshell. It's outermost electrons are 4s^2 and 3d^6. Most of the metals in this area often lose the 4s electrons.
So if Iron will lose firsts its 4s electrons, it will become Fe2+. Fe3+ happens when one of the electrons in the 4d orbitals will be lost together with the two 4s electrons. let me show you an illustration:
Fe 4s2 3d6 ll ll l l l l
when Fe2+ happens:
_ ll l l l l
* 2 electrons in 4s are lost.
when Fe3+ happens:
_ l l l l l
*another electron is lost from the 3d orbitals.
it happens because an element prefer to have half filled degeneracy.
Hope this helps :)