Electronic Configuration and Oxidation States — Revision Notes

⚡ 30-Second Revision

General configuration: $[Rn] 5f^{1-14} 6d^{0-1} 7s^2$
Exceptions: Th ($[Rn] 6d^2 7s^2$), Pa ($[Rn] 5f^2 6d^1 7s^2$), U ($[Rn] 5f^3 6d^1 7s^2$), Cm ($[Rn] 5f^7 6d^1 7s^2$)
Most common O.S.: +3 (especially later actinoids)
Variable O.S.: +3, +4, +5, +6, +7 (due to comparable $E_{5f}, E_{6d}, E_{7s}$)
Highest O.S.: +7 (Np, Pu)
Stable +6: U (as $UO_2^{2+}$)
Actinoid Contraction: Due to poor shielding of 5f electrons.

2-Minute Revision

Actinoids are f-block elements (Z=89-103) characterized by the filling of the 5f subshell. Their general electronic configuration is $[Rn] 5f^{1-14} 6d^{0-1} 7s^2$. However, key exceptions exist: Thorium (Th) is $[Rn] 6d^2 7s^2$, Protactinium (Pa) is $[Rn] 5f^2 6d^1 7s^2$, Uranium (U) is $[Rn] 5f^3 6d^1 7s^2$, and Curium (Cm) is $[Rn] 5f^7 6d^1 7s^2$ (due to half-filled 5f stability).

The most striking feature is their variable oxidation states (+3, +4, +5, +6, +7), unlike lanthanoids which are predominantly +3. This variability arises because the 5f, 6d, and 7s orbitals have very similar energy levels, allowing electrons from all three to participate in bonding.

While +3 is the most common and stable oxidation state for many actinoids (especially later ones), early actinoids like U, Np, and Pu show higher states. Neptunium and Plutonium can exhibit the highest +7 oxidation state.

Uranium is notable for its stable +6 state, often found in the uranyl ion ($UO_2^{2+}$). Poor shielding by 5f electrons causes actinoid contraction, a gradual decrease in atomic/ionic radii across the series.

5-Minute Revision

Actinoids, the second series of inner transition elements (Z=89-103), are defined by the progressive filling of the 5f subshell. Their general electronic configuration is $[Rn] 5f^{1-14} 6d^{0-1} 7s^2$.

It's crucial to remember the significant exceptions: Thorium (Th, Z=90) has $[Rn] 6d^2 7s^2$, Protactinium (Pa, Z=91) is $[Rn] 5f^2 6d^1 7s^2$, Uranium (U, Z=92) is $[Rn] 5f^3 6d^1 7s^2$, and Curium (Cm, Z=96) is $[Rn] 5f^7 6d^1 7s^2$.

These deviations occur because the 5f, 6d, and 7s orbitals are very close in energy, and slight energetic advantages or stability factors (like half-filled 5f subshell in Cm) can alter the filling order.

The most distinctive chemical property of actinoids is their wide range of oxidation states, including +3, +4, +5, +6, and +7. This contrasts sharply with lanthanoids, which almost exclusively exhibit a +3 oxidation state.

The reason for this variability in actinoids is the comparable energies of their 5f, 6d, and 7s orbitals, making all these electrons available for chemical bonding. The +3 oxidation state is generally the most common and stable for many actinoids, particularly the later ones (e.

g., Americium, Curium, Californium). However, early actinoids like Uranium, Neptunium, and Plutonium are known for their higher oxidation states. Uranium (U) commonly exhibits a stable +6 oxidation state, famously seen in the uranyl ion ($UO_2^{2+}$).

Neptunium (Np) and Plutonium (Pu) are unique in displaying the highest known oxidation state of +7 among actinoids. The stability of these higher oxidation states tends to decrease across the series, and they often act as strong oxidizing agents.

Another important phenomenon is 'actinoid contraction,' a gradual decrease in atomic and ionic radii across the series, caused by the poor shielding effect of the 5f electrons from the increasing nuclear charge.

Prelims Revision Notes

For NEET, focus on these key points regarding Actinoid Electronic Configuration and Oxidation States:

1
General Configuration: — The baseline is $[Rn] 5f^{1-14} 6d^{0-1} 7s^2$. Remember 'Rn' is the Radon core.
2
Crucial Exceptions: — Memorize the specific configurations for:

* Thorium (Th, Z=90): $[Rn] 6d^2 7s^2$ (no 5f electrons in ground state, behaves like a d-block element). * Protactinium (Pa, Z=91): $[Rn] 5f^2 6d^1 7s^2$. * Uranium (U, Z=92): $[Rn] 5f^3 6d^1 7s^2$. * Curium (Cm, Z=96): $[Rn] 5f^7 6d^1 7s^2$ (due to stable half-filled $5f^7$ configuration, similar to Gadolinium in lanthanoids).

1
Order of Electron Removal: — When forming ions, electrons are removed first from the outermost 7s orbital, then from the 6d orbital, and finally from the 5f orbital.
2
Oxidation States:

* Most Common: +3 is the most common and generally stable oxidation state for actinoids, especially from Americium (Am) onwards. * Variability: Actinoids show a wide range of oxidation states (+3, +4, +5, +6, +7).

This is a key differentiator from lanthanoids (predominantly +3). * Reason for Variability: The comparable energies of 5f, 6d, and 7s orbitals allow electrons from all three subshells to participate in bonding.

* Highest Oxidation States: Neptunium (Np) and Plutonium (Pu) exhibit the highest oxidation state of +7. * Stable +6: Uranium (U) is well-known for its stable +6 oxidation state, particularly in the uranyl ion ($UO_2^{2+}$).

* Stability Trend: The stability of higher oxidation states generally decreases across the series, and they tend to be strong oxidizing agents.

1
Actinoid Contraction: — This is the gradual decrease in atomic and ionic radii across the series. It is caused by the poor shielding effect of the 5f electrons, leading to an increased effective nuclear charge.
2
Comparison with Lanthanoids: — Be ready to compare actinoids and lanthanoids on all these points (configuration, oxidation states, magnetic properties, complex formation, radioactivity). Actinoids are all radioactive, unlike lanthanoids where only Promethium is.

Vyyuha Quick Recall

To remember the key actinoid exceptions for electronic configuration: The Paper Under Curtains. (Th, Pa, U, Cm). For oxidation states, remember: 3 is common, 7 is high (Np, Pu), 6 is U-nique (Uranium).