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Q4.8:
What are the characteristics of the transition elements and why are they called transition elements? Which of the d-block elements may not be regarded as the transition elements?

Solution :

1. Definition and Nomenclature

Transition elements are defined as elements which have incompletely filled $d$-orbitals in their ground state or in any one of their common oxidation states [Per IUPAC criteria]. The term "transition" signifies that these elements occupy the position in the Periodic Table between the $s$-block (highly electropositive metals) and the $p$-block (non-metals/metalloids), marking a transition in chemical and physical properties.

2. Characteristics of Transition Elements

  • Variable Oxidation States: Due to the small energy gap between $(n-1)d$ and $ns$ orbitals, electrons from both can participate in bonding, leading to multiple oxidation states.
  • Formation of Colored Ions: The presence of partially filled $d$-orbitals allows for $d-d$ transitions when light is absorbed, resulting in visible color [Per Crystal Field Theory].
  • Paramagnetism: Transition metal ions often possess unpaired electrons, which results in permanent magnetic moments measured in Bohr Magnetons ($\mu_{eff} = \sqrt{n(n+2)} \, BM$).
  • Catalytic Activity: Large surface area and variable oxidation states allow these metals to provide surfaces for reactant adsorption, facilitating reactions via lower activation energy paths.
  • Complex Formation: Small size, high nuclear charge, and availability of vacant $d$-orbitals allow them to act as Lewis acids and coordinate with ligands.

3. Elements Not Regarded as Transition Elements

The elements $Zn$ ($[Ar]3d^{10}4s^2$), $Cd$ ($[Kr]4d^{10}5s^2$), and $Hg$ ($[Xe]4f^{14}5d^{10}6s^2$) are generally not considered transition elements. In their elemental states, they possess a fully filled $d^{10}$ configuration. Furthermore, in their most common stable oxidation state ($M^{2+}$), the $d$-orbital remains fully filled ($d^{10}$). Consequently, they do not exhibit the characteristic properties of transition metals, such as colored ion formation or catalytic behavior arising from partially filled $d$-shells.

4. Comparative Electronic Configuration Analysis

Element Atomic Number Ground State Configuration $d$-orbital Status
$Fe$ (Typical) 26 $[Ar]3d^{6}4s^{2}$ Incompletely filled (Valid)
$Zn$ (Non-Transition) 30 $[Ar]3d^{10}4s^{2}$ Fully filled (Invalid)
$Hg$ (Non-Transition) 80 $[Xe]4f^{14}5d^{10}6s^{2}$ Fully filled (Invalid)

5. Visual Representation of d-Orbital Occupancy

Fe ($3d^6$) Transition Element: d-orbital partially filled. Zn ($3d^{10}$) Non-Transition: d-orbital fully filled.

Final Solution: Transition elements are defined by the presence of incompletely filled $d$-orbitals in their atomic or common ionic states. They are named for their bridging position between electropositive $s$-block and covalent $p$-block elements. Elements like $Zn$, $Cd$, and $Hg$ are excluded from this classification because their $d$-shells are completely filled ($d^{10}$) in both ground and $M^{2+}$ states.


More Questions from Class 12 Chemistry The d- and f-Block Elements Exercises


CBSE Solutions for Class 12 Chemistry The d- and f-Block Elements


Chapters in CBSE - Class 12 Chemistry


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