Transition metals can produce complexes or ions with a wide range of colors. The colors depend on the element and on whether or not it is dissolved in water or another solvent. Because they show the makeup of the sample, the colors are helpful in qualitative research. An element is considered to be a transition element if it can give rise to stable ions with incompletely filled d-orbitals. According to this theory, not all elements in group d of the periodic table are transition elements. A typical transition metal can exist in a wide variety of oxidation states because it lacks a partially filled d-orbital.
Elements near the center of the periodic table with occupied d orbitals are sometimes referred to as transition elements, however, they should be called d block elements instead. An element is said to be transitional if it produces one or more stable ions with only partly full d orbitals. Zn with the electronic configuration [Ar]3d104s2 is not a transition element by any definition. Its 3D orbital structure is complete. Ionization removes the 4s electrons, leaving the 3d shell full.
Why do we see different colors among transition elements?
Coloration is common in complexes that contain transition elements but not in those that do not. This demonstrates that the unoccupied d-orbitals contribute to color creation. Keep in mind that the d-orbitals of transition metals are only half full.
Since transition elements have unoccupied d-orbitals, the resulting complexes, and solutions tend to be a rainbow of hues. Since the d-orbitals are degenerate, the ions lack inherent color. They share similar spectral signals and energies, which is another way of saying they are similar. As they bond with other molecules, transition metal ions take on new colors in the process of forming complexes and compounds. Complexes are formed when one or more negatively or neutrally charged ligands bind to a transition metal. The ligand alters the d-orbital geometry. The energy levels of some d-orbitals increase while those of others decrease. This causes an energetic hoover. The (wavelength) of the absorbed photon is set by the magnitude of the energy gap.