Transition elements differ because their atoms or ions have incomplete d-sublevels. The similar energies of their d and s electrons produce characteristic properties, particularly variable oxidation states, colored complexes, and catalytic activity.
The Reasoning
Across the first-row transition elements, the 4s and 3d sublevels are close in energy. Consequently, different numbers of electrons can be removed or used in bonding without an extremely large increase in ionization energy.
| Characteristic property | Explanation | Example |
|---|---|---|
| Variable oxidation states | Similar successive ionization energies allow different numbers of 4s and 3d electrons to be removed. | Iron commonly forms and . |
| Colored complexes | Ligands cause the five d-orbitals to split into different energy levels. Visible light may be absorbed to promote an electron between these levels; the observed color is complementary to the absorbed color. | Aqueous copper(II) complexes appear blue. |
| Catalytic activity | Transition elements can change oxidation state during a reaction, forming intermediates and providing an alternative pathway with lower activation energy. Solid transition metals can also adsorb reactants onto their surfaces, weakening bonds and improving collision geometry. | Iron catalyzes ammonia production; nickel catalyzes alkene hydrogenation. |
For color, the energy absorbed satisfies . Different ligands produce different d-orbital splitting energies, so complexes of the same metal ion may have different colors.
A common misconception is that all transition-metal ions are colored. Ions with empty or full d-sublevels cannot undergo d-to-d electronic transitions and are often colorless. For example, has a full sublevel.
Exam Technique
For an IB Chemistry HL explain question, connect each property explicitly to electronic structure. Do not merely state that transition elements are colored or catalytic: mention d-orbital splitting for color, similar ionization energies for variable oxidation states, and an alternative lower-activation-energy pathway for catalysis.
