Why are Scandium(III) and Zinc(II) ions colorless in aqueous solution?

• Scandium ($\text{Sc}$) has an atomic number of 21, with an electron configuration of $[\text{Ar}] 3d^1 4s^2$. When it forms the $\text{Sc}^{3+}$ ion, it loses all three valence electrons, resulting in an electron configuration of $[\text{Ar}] 3d^0$. Since there are no d-electrons, no d-d transitions can occur.

• Zinc ($\text{Zn}$) has an atomic number of 30, with an electron configuration of $[\text{Ar}] 3d^{10} 4s^2$. When it forms the $\text{Zn}^{2+}$ ion, it loses its two $4s$ electrons, resulting in an electron configuration of $[\text{Ar}] 3d^{10}$. Since the d-orbitals are completely filled, there are no empty d-orbitals for electrons to be promoted to, so no d-d transitions can occur.

Because both $\text{Sc}^{3+}$ and $\text{Zn}^{2+}$ lack partially filled d-orbitals, they cannot undergo d-d transitions and therefore appear colorless.