Scandium is one of the two elements in the first transition metal period which has only one oxidation state (zinc is the other, with an oxidation state of +2).

On the basis of incompletely filled 3d orbitals, scandium  $(\text{Z}=21)$has incompletely filled d-orbitals$(3{\text{d}}^1)$  in its ground state, so it is regarded as a transition element.

On the other hand, zinc$(\text{Z}=30)$has completely filled d-orbitals  $(3{\text{d}}^{10})$  in its ground state.