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copper displays two oxidation states (+1 and +2). In the +1 oxidation state, an electron is removed from the s-orbital. However, in the +2 oxidation state, an electron is removed from the d-orbital. Thus, the d-orbital now becomes incomplete (3d⁹). Hence, it is a transition element. wherein it will have incompletely filled d-orbitals (4d), hence a transition metal.

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The melting points of transition elements are high due to the presence of strong intermetallic bonds (formed by valence electrons) and covalent bonds (formed due to d-d overlapping of unpaired d-electrons).

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Zn, Cd, Hg are not regarded as transition element Because they have the completely filled d-sub-shell with outer electronic configuration (n-1)d¹⁰ns^2.

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On the basis of incompletely filled 3d orbitals in case of scandium atom is its ground state (3d¹), it is regarded as a transition element. On the other hand, zinc atom has completely filled dorbitals (3d¹⁰) in its ground state as well as in its oxidised state, hence it is not regarded as a transition elements.

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Scandium (Sc).