d-and f-Block Elements - Electronic Configurations and General Properties of Transition Elements

Definition: Transition elements are defined as those elements which have incompletely filled $d$-orbitals in their ground state or in any of their oxidation states. Elements like $Zn$, $Cd$, and $Hg$ of group 12 have full $d^{10}$ configuration and are technically not transition elements, though they are studied with them.

Electronic Configuration: The general electronic configuration of $d$-block elements is $(n-1)d^{1-10} ns^{1-2}$. Exceptional configurations occur in $Cr$ ($3d^5 4s^1$) and $Cu$ ($3d^{10} 4s^1$) due to the extra stability of half-filled and fully-filled $d$-orbitals.

Atomic and Ionic Radii: Generally, radii decrease with increasing atomic number in a series due to poor shielding by $d$-electrons and increased nuclear charge. However, the radii of the $4d$ and $5d$ series are nearly identical due to 'Lanthanoid Contraction'.

Ionization Enthalpy: There is a general increase in ionization enthalpy across each series due to an increase in nuclear charge. Irregularities arise from the varying stability of $d^0$, $3d^5$, and $3d^{10}$ configurations.

Oxidation States: Transition elements exhibit variable oxidation states due to the small energy difference between $(n-1)d$ and $ns$ orbitals. $Mn$ shows the maximum number of oxidation states (from $+2$ to $+7$) in the $3d$ series.

Magnetic Properties: Most transition metal ions are paramagnetic due to the presence of unpaired electrons. Paramagnetism increases with the number of unpaired electrons.

Formation of Colored Ions: Color is attributed to $d-d$ transitions. When light falls on the ion, electrons from lower energy $d$-orbitals are excited to higher energy $d$-orbitals, absorbing specific wavelengths. $d^0$ and $d^{10}$ ions (like $Sc^{3+}$ and $Zn^{2+}$) are typically colorless.

Catalytic Properties: Transition metals and their compounds act as catalysts because they can adopt multiple oxidation states and form unstable intermediate complexes (e.g., $V_2O_5$ in the Contact Process, $Fe$ in Haber's Process).

Interstitial Compounds: These are formed when small atoms like $H$, $C$, or $N$ are trapped inside the crystal lattices of transition metals. They are non-stoichiometric, hard, and retain metallic conductivity.