Lanthanoids — Revision Notes

⚡ 30-Second Revision

Definition: — Ce (Z=58) to Lu (Z=71), 4f-block elements.
Electronic Config: — $[Xe] 4f^{1-14} 5d^{0-1} 6s^2$.
Oxidation State: — Predominant +3. Exceptions: +2 ($Eu^{2+}$ ($4f^7$), $Yb^{2+}$ ($4f^{14}$), $Sm^{2+}$ ($4f^6$)), +4 ($Ce^{4+}$ ($4f^0$), $Tb^{4+}$ ($4f^7$), $Pr^{4+}$ ($4f^1$)).
Lanthanoid Contraction: — Gradual decrease in atomic/ionic radii ($Ln^{3+}$) from Ce to Lu.

- Cause: Poor shielding of 4f electrons. - Consequences: Similar size of 2nd & 3rd transition series (Zr/Hf), decreasing basicity of $Ln(OH)_3$.

Magnetic Properties: — Most are paramagnetic (unpaired 4f electrons). $La^{3+}$ ($4f^0$) & $Lu^{3+}$ ($4f^{14}$) are diamagnetic. Both spin & orbital contributions are significant.
Colour: — Pale colours due to f-f transitions.
Mischmetal: — Alloy of ~95% Lanthanoids (Ce, La, Nd, Pr) + ~5% Fe. Used in lighter flints.

2-Minute Revision

Lanthanoids, elements from Cerium (Ce, Z=58) to Lutetium (Lu, Z=71), are f-block elements characterized by the filling of the 4f subshell. Their general electronic configuration is $[Xe] 4f^{1-14} 5d^{0-1} 6s^2$.

The most stable oxidation state is +3, but some exhibit +2 or +4 states to achieve stable $4f^0$ (e.g., $Ce^{4+}$), $4f^7$ (e.g., $Eu^{2+}$, $Gd^{3+}$, $Tb^{4+}$), or $4f^{14}$ (e.g., $Yb^{2+}$, $Lu^{3+}$) configurations.

A key phenomenon is 'lanthanoid contraction,' the gradual decrease in atomic and ionic radii across the series, caused by the poor shielding of 4f electrons. This contraction leads to crucial consequences: similar atomic radii for elements of the 2nd and 3rd transition series (e.

g., Zr and Hf), and a decrease in the basicity of lanthanoid hydroxides from $La(OH)_3$ to $Lu(OH)_3$. Most lanthanoid ions are paramagnetic due to unpaired 4f electrons, with $La^{3+}$ and $Lu^{3+}$ being diamagnetic.

Their colours are generally pale, arising from f-f electronic transitions. Mischmetal, an alloy of lanthanoids and iron, is a notable application, used in lighter flints.

5-Minute Revision

Lanthanoids are the 14 elements from Cerium (Ce, Z=58) to Lutetium (Lu, Z=71), forming the first series of f-block elements. Their defining characteristic is the progressive filling of the 4f orbitals.

The general electronic configuration is $[Xe] 4f^{1-14} 5d^{0-1} 6s^2$, with exceptions like Gadolinium (Gd) and Lutetium (Lu) having a $5d^1$ electron in their ground state. The most common and stable oxidation state for all lanthanoids is +3, formed by losing the two $6s$ electrons and one electron from either $5d$ or $4f$.

However, some exhibit +2 or +4 states, which are stabilized by achieving empty ($4f^0$), half-filled ($4f^7$), or fully-filled ($4f^{14}$) 4f subshells. For example, $Ce^{4+}$ ($4f^0$) is a strong oxidizing agent, while $Eu^{2+}$ ($4f^7$) is a strong reducing agent.

The most important concept is Lanthanoid Contraction. This refers to the steady decrease in atomic and ionic radii (especially $Ln^{3+}$ ions) as we move from Ce to Lu. It's caused by the poor shielding effect of the 4f electrons.

As the nuclear charge increases across the series, the 4f electrons don't effectively shield the outer electrons, leading to a stronger pull from the nucleus and a reduction in size. Consequences include: (1) The atomic radii of elements in the 2nd and 3rd transition series become very similar (e.

g., Zr and Hf), making them chemically alike. (2) The basicity of lanthanoid hydroxides, $Ln(OH)_3$, decreases from $La(OH)_3$ to $Lu(OH)_3$ because smaller ionic size increases covalent character.

Most lanthanoid ions are paramagnetic due to the presence of unpaired electrons in their 4f orbitals. The 4f orbitals are deeply buried, so orbital angular momentum contributes significantly to the magnetic moment.

Exceptions are $La^{3+}$ ($4f^0$) and $Lu^{3+}$ ($4f^{14}$), which are diamagnetic. Many lanthanoid ions are coloured, typically pale, due to f-f electronic transitions. A key application is Mischmetal, an alloy (95% lanthanoids, 5% Fe) used in lighter flints due to its pyrophoric nature.

Prelims Revision Notes

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Definition & Position: — Lanthanoids are 14 elements (Ce to Lu, Z=58-71) in the f-block, following Lanthanum (La). They are also called 'rare earth elements' (a misnomer, as many are abundant).
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Electronic Configuration: — General form: $[Xe] 4f^{1-14} 5d^{0-1} 6s^2$. Important exceptions: Gd ($[Xe] 4f^7 5d^1 6s^2$) and Lu ($[Xe] 4f^{14} 5d^1 6s^2$) have a $5d^1$ electron.
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Oxidation States:

* Most common and stable: +3 (loss of $6s^2$ and one $5d/4f$ electron). * +2 states: $Eu^{2+}$ ($4f^7$), $Yb^{2+}$ ($4f^{14}$), $Sm^{2+}$ ($4f^6$). $Eu^{2+}$ and $Yb^{2+}$ are stable due to half-filled/fully-filled f-orbitals.

$Sm^{2+}$ is less stable. * +4 states: $Ce^{4+}$ ($4f^0$), $Pr^{4+}$ ($4f^1$), $Tb^{4+}$ ($4f^7$). $Ce^{4+}$ is stable due to empty f-orbital. $Tb^{4+}$ is stable due to half-filled f-orbital. * $Ce^{4+}$ is a strong oxidizing agent; $Eu^{2+}$ is a strong reducing agent.

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Lanthanoid Contraction:

* Definition: Gradual decrease in atomic and ionic radii ($Ln^{3+}$) from Ce to Lu. * Cause: Poor shielding effect of 4f electrons. As nuclear charge increases, 4f electrons do not effectively shield outer electrons, leading to increased effective nuclear charge and smaller size.

* Consequences: * Similar Radii: Elements of 2nd and 3rd transition series have similar atomic radii (e.g., Zr ($160,\text{pm}$) and Hf ($159,\text{pm}$)). * Basicity of Hydroxides: Basicity of $Ln(OH)_3$ decreases from $La(OH)_3$ to $Lu(OH)_3$ (due to increased covalent character with decreasing $Ln^{3+}$ size).

* Slight increase in ionization energy and electronegativity.

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Magnetic Properties:

* Most $Ln^{3+}$ ions are paramagnetic due to unpaired 4f electrons. * $La^{3+}$ ($4f^0$) and $Lu^{3+}$ ($4f^{14}$) are diamagnetic (no unpaired electrons). * Both spin and orbital angular momenta contribute to the magnetic moment (orbital contribution is not quenched due to shielded 4f orbitals).

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Colour: — Many $Ln^{3+}$ ions are coloured (generally pale) due to f-f transitions. These transitions are sharp and narrow.
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Complex Formation: — Low tendency to form complexes compared to d-block elements.
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Applications:

* Mischmetal: Alloy of ~95% lanthanoids (Ce, La, Nd, Pr) + ~5% Fe. Used in lighter flints (pyrophoric). * Cerium oxide: Polishing agent for glass. * Nd:YAG lasers, phosphors (Eu, Tb), strong magnets (SmCo, NdFeB).

Vyyuha Quick Recall

To remember the common stable oxidation states and their configurations:

Clever Elephants Yell Loudly, Getting Tired.

Ce ($Ce^{4+}$) $ightarrow 4f^0$ (empty)
Eu ($Eu^{2+}$) $ightarrow 4f^7$ (half-filled)
Yb ($Yb^{2+}$) $ightarrow 4f^{14}$ (fully-filled)
Lu ($Lu^{3+}$) $ightarrow 4f^{14}$ (fully-filled, diamagnetic)
Gd ($Gd^{3+}$) $ightarrow 4f^7$ (half-filled)
Tb ($Tb^{4+}$) $ightarrow 4f^7$ (half-filled)

This helps recall the key elements that show stable +2 or +4 states due to $4f^0, 4f^7, 4f^{14}$ configurations, and the diamagnetic $Lu^{3+}$. Remember the general +3 state for all others.