The Periodic Table - Transition elements

A transition element is defined as a d-block element that forms at least one stable ion with a partially filled $d$ subshell. This explains why $Sc$ (forms $Sc^{3+}$ with $3d^0$) and $Zn$ (forms $Zn^{2+}$ with $3d^{10}$) are sometimes excluded from the strict definition.

Physical properties include high melting points, high densities, and high tensile strength compared to s-block metals due to the involvement of $3d$ and $4s$ electrons in metallic bonding.

Variable oxidation states occur because the energy levels of the $4s$ and $3d$ subshells are very close, allowing varying numbers of electrons to be lost. For example, Manganese ($Mn$) can exhibit oxidation states from $+2$ to $+7$.

Complex ions consist of a central metal cation bonded to ligands via coordinate (dative covalent) bonds. Ligands like $H_2O$, $NH_3$, and $CN^-$ possess at least one lone pair of electrons.

The characteristic colors of transition metal complexes arise from the splitting of $d$ orbitals into two non-degenerate energy levels ($t_{2g}$ and $e_g$). Electrons absorb specific frequencies of visible light to jump from lower to higher energy levels ($d$-$d$ transitions).

Transition metals act as catalysts because they can provide a surface for adsorption (heterogeneous) or change oxidation states to facilitate electron transfer (homogeneous). Examples include $Fe$ in the Haber Process and $V_2O_5$ in the Contact Process.