Periodicity - First-row d-block elements (HL only)

A transition element is defined as an element that possesses an incomplete d-subshell in one or more of its stable oxidation states. $Sc$ and $Zn$ are often excluded as $Sc^{3+}$ has a $3d^0$ configuration and $Zn^{2+}$ has a $3d^{10}$ configuration.

Transition metals exhibit variable oxidation states because the energy levels of the $4s$ and $3d$ subshells are very close. Common states include $+2$ (loss of $4s$ electrons) and higher states involving $3d$ electrons.

Complex ions consist of a central metal ion surrounded by ligands. Ligands are species with a lone pair of electrons that form coordinate covalent bonds with the metal ion (e.g., $H_2O$, $NH_3$, $Cl^-$, $CN^-$).

Magnetic properties: Paramagnetism is caused by unpaired electrons which are attracted into a magnetic field. Diamagnetism is a property of all materials where paired electrons create a weak opposition to a magnetic field. Ferromagnetism (e.g., $Fe$, $Co$, $Ni$) is a permanent magnetic effect.

The origin of color in transition metal complexes is due to the splitting of the d-orbitals into two sets of non-degenerate energy levels in the presence of ligands (Crystal Field Theory). The energy difference is denoted as $\Delta E$.

Light is absorbed when an electron transitions from a lower energy d-orbital to a higher energy d-orbital (d-d transition). The color observed is the complementary color to the wavelength absorbed.

Factors affecting the color of complexes include the identity of the metal ion, the oxidation state of the metal, the geometry of the complex, and the nature of the ligand (as ordered in the spectrochemical series).

Transition metals and their compounds function as catalysts. Heterogeneous catalysts (e.g., $Fe$ in the Haber process, $V_2O_5$ in the Contact process) are in a different phase than reactants; homogeneous catalysts are in the same phase.