The elements having d-orbital as penultimate orbital and the outermost s-orbital, like transition metals, show variable valency.
In other words, transition elements show a variable valancy due to involvement of d-orbitlas electron in bonding.
For example :- Mn can show 2 3,4,5,6,7 oxidation states.
Explanation:
This is because they have unfilled d-orbital but filled s-orbital.
For example Iron (Fe) Atomic no 26, so 26 electrons to be filled in different orbital. So the electronic configuration is
(1s)2 (2s)2 (2p)6 (3s)2 (3p)6 (4s)2 (3d)6 this is the way of filling of electrons according to aufbau's principle. But the exact arrangement is (1s)2 (2s)2 (2p)6 (3s)2 (3p)6 (3d)6 (4s)2
So Iron can lose two electrons from the outermost 4s subshell to show a valency of two. Then it can lose another electron from the 3d subshell (penultimate) two show a valency of three. When it shows the valency of two the electron configuration is (1s)2 (2s)2 (2p)6 (3s)2 (3p)6 (3d)6 (4s)0
When it shows the valency of two the electron configuration is (1s)2 (2s)2 (2p)6 (3s)2 (3p)6 (3d)5 (4s)0
And the +3 valence state is stabler than the +2 valence state, because the d-orbital contains 5 electrons (halffilled). As you know fully filled and exactly half filled orbitals are more stable than the orbitals having different no of electrons, i.e. less than half filled and more than half filled but not fully filled.
The transition metals show variable oxidation states. like manganese, Mn shows +2, +4, +6, +7 oxidation states.
