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Q4.20(i):
Compare the chemistry of actinoids with that of the lanthanoids with special reference to: (i) electronic configuration

Solution :

1. Theoretical Foundation: Electronic Configurations

The electronic configurations of lanthanoids and actinoids are governed by the filling of $4f$ and $5f$ orbitals, respectively. Both series represent the inner transition elements, but they exhibit distinct differences due to the energy levels and radial extensions of their respective $f$-orbitals.

2. Lanthanoid Electronic Configuration

Lanthanoids are defined by the filling of the $4f$ subshell. The general valence shell electronic configuration is represented as:

$[Xe] 4f^{1-14} 5d^{0-1} 6s^2$

[Per the Aufbau Principle, the $4f$ orbital is buried deep within the atom and does not participate significantly in chemical bonding.]

3. Actinoid Electronic Configuration

Actinoids involve the filling of the $5f$ subshell. Due to the small energy difference between the $5f$, $6d$, and $7s$ orbitals, the electrons are more distributed, leading to a more complex configuration:

$[Rn] 5f^{0-14} 6d^{0-2} 7s^2$

[Per the screening effect theory, the $5f$ electrons provide poorer shielding than $4f$ electrons, resulting in different ionization energies.]

4. Comparative Tabulation

Feature Lanthanoids ($4f$) Actinoids ($5f$)
Orbital Energy $4f$ electrons are more deeply buried. $5f$ electrons extend further from the nucleus.
Binding Energy Higher binding energy. Lower binding energy ($5f$ are less tightly held).
Hybridization/Bonding Rarely participate in bonding. Frequently participate in chemical bonding.

5. Visual Representation of Orbital Energy Levels

Relative Energy Levels 4f (Lanthanoids) 5f (Actinoids) Higher energy accessibility

6. Analytical Conclusion

The core distinction in electronic configuration lies in the radial extension of the orbitals. In lanthanoids, the $4f$ electrons are shielded effectively by $5s$ and $5p$ shells, rendering them chemically inert. Conversely, in actinoids, the $5f$ orbitals have greater radial extension and the $5f$, $6d$, and $7s$ orbitals have comparable energies, allowing the $5f$ electrons to participate in covalent bonding and facilitating a wider range of oxidation states compared to the lanthanoids.

Final Solution: While lanthanoids are restricted to filling the $4f$ orbital with a general configuration of $[Xe] 4f^{1-14} 5d^{0-1} 6s^2$, actinoids exhibit a more complex, variable configuration $[Rn] 5f^{0-14} 6d^{0-2} 7s^2$ due to the comparable energy levels of the $5f, 6d, 7s$ subshells, leading to greater chemical diversity in actinoids.


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