d and f Block Elements - Characteristics of transition metals

Transition elements are defined as elements having a partially filled $d$-subshell in their ground state or in any of their common oxidation states. $Zn$, $Cd$, and $Hg$ are generally not considered transition metals as they have a full $d^{10}$ configuration.

General Electronic Configuration: $(n-1)d^{1-10} ns^{1-2}$. Notable exceptions include $Cr$ ($3d^5 4s^1$) and $Cu$ ($3d^{10} 4s^1$) due to the extra stability of half-filled and fully-filled $d$-orbitals.

Atomic and Ionic Radii: There is a general decrease in atomic radii across a series due to an increase in nuclear charge, though the decrease is small because of the shielding effect of $(n-1)d$ electrons. The similarity in sizes of $4d$ and $5d$ series elements (e.g., $Zr$ and $Hf$) is due to 'Lanthanoid Contraction'.

Variable Oxidation States: Transition metals show multiple oxidation states because the energy difference between $(n-1)d$ and $ns$ orbitals is very small. For example, Manganese ($Mn$) shows oxidation states from $+2$ to $+7$.

Magnetic Properties: Most transition metal ions are paramagnetic due to the presence of unpaired electrons. The magnetic moment is calculated using the 'spin-only' formula.

Formation of Colored Ions: The color of transition metal ions is attributed to $d-d$ transitions. When light falls on an ion, an electron from a lower energy $d$-orbital is excited to a higher energy $d$-orbital by absorbing a specific wavelength of visible light.

Catalytic Properties: Transition metals and their compounds (like $V_2O_5$, $Fe$, $Ni$) act as good catalysts due to their ability to show variable oxidation states and their ability to form unstable intermediate complexes.

Formation of Interstitial Compounds: Transition metals form interstitial compounds by trapping small atoms like $H$, $C$, or $N$ in the vacant spaces (interstices) of their crystal lattices. These compounds are hard and have high melting points.