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Why do iron ions have charges of +2 or +3?

Iron is a transition metal and has an atomic number of 26. Its electronic configuration can be written as:


`1s^2, 2s^2, 2p^6, 3s^2, 3p^6, 3d^6, 4s^2`


As one can see, the 4s orbital is fully filled, while the 3d orbital has 6 electrons. These 6 electrons are distributed in such a way that only 1 electron pair exists; all the other electrons are unpaired. 


Also note that 3d and 4s orbitals are very close to each other in terms of energy levels. Thus, the 4s orbital loses electrons first. When the 4s orbital of iron loses the 2 electrons, a valency of 2 is obtained. When the atom loses the two 4s electrons and 1 electron from the 3d orbital (leaving the other 5 electrons unpaired and the 3d orbital half filled, thereby lending more stability to it), a valency of 3 results.


Thus, we have either a +2 charge or a +3 charge on the iron ions.


Hope this helps.  

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