Oxidation States and Lanthanoid Contraction — Revision Notes

⚡ 30-Second Revision

Common Oxidation State: — +3 for most lanthanoids.
Exceptional Oxidation States:

- +2: Eu ($[Xe]4f^7$), Yb ($[Xe]4f^{14}$) - strong reducing agents. - +4: Ce ($[Xe]4f^0$), Tb ($[Xe]4f^7$) - strong oxidizing agents.

Lanthanoid Contraction: — Gradual decrease in atomic/ionic radii (La to Lu).
Cause: — Poor shielding effect of 4f electrons.
Consequences:

1. Similar radii of 4d and 5d elements (e.g., Zr $approx$ Hf). 2. Increased density of 5d elements. 3. Decreasing basicity of Ln(OH)$_3$ (La to Lu).

2-Minute Revision

Lanthanoids, elements from Ce to Lu, predominantly exhibit a +3 oxidation state, formed by losing two 6s and one 5d/4f electron. However, specific lanthanoids like Europium (Eu) and Ytterbium (Yb) show a +2 state, achieving stable half-filled (f$^7$) or fully-filled (f$^{14}$) configurations, making them strong reducing agents.

Cerium (Ce) and Terbium (Tb) can display a +4 state, aiming for stable empty (f$^0$) or half-filled (f$^7$) configurations, making them strong oxidizing agents. The defining characteristic is 'lanthanoid contraction,' a steady decrease in atomic and ionic radii across the series.

This is caused by the poor shielding of the inner 4f electrons, leading to an increased effective nuclear charge pulling outer electrons closer. Consequences include the striking similarity in size and properties of 4d and 5d transition elements (e.

g., Zr and Hf), higher densities of 5d elements, and a decrease in the basicity of lanthanoid hydroxides from La to Lu.

5-Minute Revision

The lanthanoids are a series of 14 elements (Ce-Lu) characterized by the filling of the 4f subshell. Their general electronic configuration is $[Xe] 4f^{1-14} 5d^{0-1} 6s^2$. The most common and stable oxidation state for these elements is +3, resulting from the loss of the two 6s electrons and one 5d or 4f electron. For example, for Gadolinium (Gd, $[Xe] 4f^7 5d^1 6s^2$), Gd$^{3+}$ is $[Xe] 4f^7$.

However, some lanthanoids deviate from this +3 state to achieve particularly stable f-electron configurations: f$^0$ (empty), f$^7$ (half-filled), or f$^{14}$ (fully-filled). For instance, Europium (Eu, $[Xe] 4f^7 6s^2$) forms Eu$^{2+}$ ($[Xe] 4f^7$), which is a stable half-filled configuration, making Eu$^{2+}$ a strong reducing agent.

Similarly, Ytterbium (Yb, $[Xe] 4f^{14} 6s^2$) forms Yb$^{2+}$ ($[Xe] 4f^{14}$), a stable fully-filled configuration, also acting as a reducing agent. Cerium (Ce, $[Xe] 4f^1 5d^1 6s^2$) forms Ce$^{4+}$ ($[Xe] 4f^0$), a stable empty configuration, making Ce$^{4+}$ a powerful oxidizing agent.

Terbium (Tb) can also show a +4 state (Tb$^{4+}$ is f$^7$).

The 'lanthanoid contraction' is a crucial phenomenon: a gradual decrease in atomic and ionic radii across the lanthanoid series (La to Lu). This occurs because the 4f electrons, being deeply embedded and diffuse, provide very poor shielding to the outer electrons from the increasing nuclear charge.

This results in a higher effective nuclear charge, pulling the electron cloud closer to the nucleus. The consequences are significant: (1) The atomic radii of 4d and 5d transition elements in the same group become nearly identical (e.

g., Zr and Hf), leading to similar chemical properties. (2) The densities of 5d elements are significantly higher than their 4d counterparts due to similar size but greater mass. (3) The basicity of lanthanoid hydroxides, Ln(OH)$_3$, decreases from La(OH)$_3$ to Lu(OH)$_3$ because the smaller Ln$^{3+}$ ions have higher charge density, making the Ln-OH bond more covalent.

Prelims Revision Notes

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Lanthanoids: — Elements from Cerium (Ce, Z=58) to Lutetium (Lu, Z=71). They are f-block elements.
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General Electronic Configuration: — $[Xe] 4f^{1-14} 5d^{0-1} 6s^2$.
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Predominant Oxidation State: — +3. This is the most stable state for most lanthanoids, formed by losing two 6s electrons and one 5d or 4f electron.
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Exceptional Oxidation States:

* +2 State: Exhibited by Europium (Eu$^{2+}$: $[Xe] 4f^7$) and Ytterbium (Yb$^{2+}$: $[Xe] 4f^{14}$). These achieve stable half-filled or fully-filled f-subshells. Eu$^{2+}$ and Yb$^{2+}$ are strong reducing agents as they tend to lose an electron to form the more common +3 state.

* +4 State: Exhibited by Cerium (Ce$^{4+}$: $[Xe] 4f^0$) and sometimes Terbium (Tb$^{4+}$: $[Xe] 4f^7$). These achieve stable empty or half-filled f-subshells. Ce$^{4+}$ is a powerful oxidizing agent as it readily gains an electron to form Ce$^{3+}$.

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Lanthanoid Contraction: — The steady, gradual decrease in atomic and ionic radii (specifically for Ln$^{3+}$ ions) across the lanthanoid series from La to Lu.
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Cause of Lanthanoid Contraction: — Poor shielding effect of 4f electrons. As nuclear charge increases across the series, the 4f electrons are ineffective at screening the outer electrons, leading to an increased effective nuclear charge and a stronger pull on valence electrons.
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Consequences of Lanthanoid Contraction:

* Similarity of 4d and 5d Transition Elements: Elements in the same group of 4d and 5d series (e.g., Zirconium (Zr) and Hafnium (Hf)) have nearly identical atomic and ionic radii, leading to very similar chemical properties.

* Increased Density of 5d Elements: Due to similar size but higher atomic mass, 5d elements have significantly higher densities than their 4d counterparts. * Decreasing Basicity of Hydroxides: The basicity of Ln(OH)$_3$ decreases from La(OH)$_3$ to Lu(OH)$_3$.

Smaller Ln$^{3+}$ ions have higher charge density, making the Ln-OH bond more covalent and less prone to releasing OH$^-$ ions. * Difficulty in Separation: Similar properties due to similar sizes make separation of lanthanoids challenging.

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Color: — Many Ln$^{3+}$ ions are colored due to f-f transitions. Ions with f$^0$, f$^7$, or f$^{14}$ configurations are generally colorless.

Vyyuha Quick Recall

For Lanthanoid Oxidation States: Clever Elephants Yell So Terribly.

Ce: +4 (f$^0$) - Oxidizing
Eu: +2 (f$^7$) - Reducing
Yb: +2 (f$^{14}$) - Reducing
Sm: +2 (f$^6$) - Reducing
Tb: +4 (f$^7$) - Oxidizing

For Lanthanoid Contraction Consequences: Size Density Basicity

Size: 4d/5d elements similar
Density: 5d elements higher
Basicity: Ln(OH)$_3$ decreases