Transition metals show variable valency and these elements have d-orbital as penultimate orbital and the outermost orbital is the s-orbital. Now, the atomic number of Fe, Iron is 26 and the electronic configuration is (1s)2 (2s)2 (2p)6 (3s)2 (3p)6 (4s)2 (3d)6 (aufbau's principle). But the exact arrangement is also possible with (1s)2 (2s)2 (2p)6 (3s)2 (3p)6 (3d)6 (4s)2. Therefore, because of the different electronic configurations which the element can have it shows variable valency. Another reason is that an atom has to complete 2 or 8 electrons in its outermost shell therefore, when Fe or any other transition element reacts with the other element then these transition atoms share the electrons according to their requirements.
