Scandium (symbol Sc) is a chemical element with the atomic number 21. It is the very first element in the 'd-block' (transition metals) of the periodic table, making its electron configuration an important learning step in Chemistry.
Because Scandium usually only forms the Sc³⁺ ion (where its d-orbital becomes completely empty), some chemists debate whether it should truly be classified as a typical 'transition metal', since typical transition metals have partially filled d-orbitals in their ions.
Since Scandium has 21 electrons, they are distributed into the energy levels as follows:
Full Configuration (spdf notation): 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹
Shorthand (Noble Gas) Configuration: [Ar] 4s² 3d¹ (This shorthand is used because the first 18 electrons perfectly match the electron configuration of the noble gas Argon [Ar]).
According to the Aufbau Principle (and the n+l rule), electrons fill the lowest energy orbitals first. The 4s orbital has slightly lower energy than the 3d orbital. Therefore, the 4s orbital fills up first with 2 electrons (reaching element 20, Calcium), and the 21st electron of Scandium enters the 3d orbital.
When Scandium forms bonds, it usually loses 3 electrons to become the Sc³⁺ ion. It loses the two 4s electrons and the one 3d electron, leaving it with the stable electron configuration of Argon: [Ar].
The most common and stable valency (oxidation state) of Scandium is **+3**.
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