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Why does scandium has +2 oxidation state unstable? It only has +3 oxidation state stable. Scandium has outermost electronic configuration as 3d¹4s². Why it does not lose two 4s e- only instead of all three?

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Scandium actually does have oxidation states lower than +3. Wikipedia reports a double chloride $\ce{CsScCl3}$ where scandium is clearly in the oxidation state +2.

The stability of oxidation states in transition metals depends on the balance between ionization energy on the one hand, and binding energy due to either ionic or covalent bonds on the other. In the case of scandium the third ionization energy is low because all three valence electrons are held rather loosely, being in diffuse orbitals that are shielded from most of the nuclear charge by the argon core. Thus it's easy to get from the +2 to the +3 oxidation state because of the greater binding energy enabled by the latter. Later first-row transition elements like iron have more tightly bound $3d$ electrons and thus higher third ionization energies, making the +2 oxidation state more favorable.

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