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Q4.37:
Comment on the statement that elements of the first transition series possess many properties different from those of heavier transition elements.

Solution :

1. Electronic Configuration and Atomic Size

The first transition series (3d-series: $Sc$ to $Zn$) exhibits significant differences from the 4d and 5d series due to the lack of filled $f$-orbitals. In the 3d series, the $d$-electrons are more exposed to the periphery of the atom, whereas in the 4d and 5d series, the intervening $f$-orbitals provide poor shielding [Per Slater's Rules regarding Screening Effect].

Consequently, the atomic radii of the 4d and 5d series elements are nearly identical (the Lanthanoid Contraction), whereas the 3d series shows a distinct trend of decrease followed by an increase. This disparity impacts the metallic bonding strength and melting points.

2. Comparative Analysis of Stability and Enthalpies

The enthalpy of atomization for the 3d series is lower compared to the 4d and 5d series. This is attributed to the fact that $d-d$ overlap is much more effective in the 4d and 5d series than in the 3d series [Per Molecular Orbital Theory applied to Metallic Bonding].

Property First Transition Series (3d) Heavier Series (4d/5d)
Enthalpy of Atomization Lower Higher
Ionization Enthalpy Lower Higher
Stable Oxidation States Varied (e.g., $+2, +3$ are common) Higher oxidation states are more stable

3. Chemical Reactivity and Coordination Chemistry

Step 1: Oxidation States: In the 3d series, lower oxidation states are more stable, whereas, in the 4d and 5d series, higher oxidation states are more stable (e.g., $W(VI)$ is more stable than $Cr(VI)$). This is because the bond energies for transition metal-ligand bonds are higher for the 4d and 5d series due to better spatial extension of $4d/5d$ orbitals.

Step 2: Magnetic and Spectral Properties: The 3d series exhibits more pronounced crystal field splitting energies ($\Delta_o$). However, the spin-orbit coupling is significantly higher in the 4d and 5d series, leading to distinct magnetic behaviors not observed in the 3d series [Per Russell-Saunders Coupling].

4. Visual Representation of d-Orbital Extension

3d Orbital (Compact) 4d/5d Orbital (Extended)

5. Conclusion

The fundamental differences arise from:

  • The absence of $f$-orbitals in the 3d series, eliminating lanthanoid contraction effects.
  • The higher effective nuclear charge and poorer shielding of $4f$ electrons in heavier series.
  • Increased metal-metal bonding in the 4d/5d series due to higher spatial extension of $d$-orbitals, leading to higher melting points and higher oxidation state stability.

Final Solution: The elements of the first transition series differ from heavier series due to the lack of inner $f$-orbital occupancy, smaller atomic radii, lower enthalpy of atomization, and lower stability of higher oxidation states compared to the 4d and 5d series.


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


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