Lanthanoids & Actinoids
Explore the Inner Transition Metals. Master the f-block electronic anomalies, the immense consequences of the Lanthanoid contraction, and the highly-tested +2 and +4 oxidation states.
Module Focus: The Inner Transition Elements
The f-block elements consist of two series lying at the bottom of the periodic table: the Lanthanoids (4f series, Ce to Lu) and the Actinoids (5f series, Th to Lr). Because the energy difference between $(n-2)f$, $(n-1)d$, and $ns$ orbitals is incredibly small, electrons frequently jump between them, creating a slew of electronic configuration anomalies that are heavily tested in NEET.
1. Electronic Configuration & Anomalies
The general electronic configuration of f-block elements is $(n-2)f^{1-14} (n-1)d^{0-1} ns^2$. Notice that the d-orbital usually contains zero electrons, but occasionally grabs one to provide extra stability to the f-subshell.
| Element | Atomic No. (Z) | Configuration |
|---|---|---|
| Lanthanum (La) | 57 | $[Xe] \ 4f^0 \ 5d^1 \ 6s^2$ (Technically a d-block element, but studied here) |
| Gadolinium (Gd) | 64 |
$[Xe] \ \mathbf{4f^7 \ 5d^1} \ 6s^2$ (Electron enters 5d to preserve the ultra-stable half-filled $4f^7$ core) |
| Lutetium (Lu) | 71 |
$[Xe] \ \mathbf{4f^{14} \ 5d^1} \ 6s^2$ (Electron enters 5d because 4f is completely full) |
2. Oxidation States: The $+3$ Rule and Exceptions
The most characteristic and stable oxidation state of ALL lanthanoids and actinoids is $+3$. However, some elements show $+2$ or $+4$ states if it leads to an empty ($f^0$), half-filled ($f^7$), or fully-filled ($f^{14}$) subshell.
Cerium ($Z=58$) easily loses 4 electrons to attain a noble gas core ($[Xe] \ 4f^0$).
NEET Trap:
Even though $Ce^{4+}$ is stable, $+3$ is the ultimate stable state for the series. Therefore, $Ce^{4+}$ strongly desires to gain an electron to become $Ce^{3+}$. Thus, $Ce^{4+}$ acts as a powerful Oxidizing Agent (it gets reduced itself).
Europium ($Z=63$) loses 2 electrons to attain a highly stable half-filled configuration ($[Xe] \ 4f^7$).
NEET Trap:
Because $+3$ is the most stable state for the entire series, $Eu^{2+}$ strongly desires to lose one more electron to become $Eu^{3+}$. Thus, $Eu^{2+}$ acts as a strong Reducing Agent (it gets oxidized itself).
3. The Lanthanoid Contraction
As we move from Lanthanum ($La$) to Lutetium ($Lu$), there is a steady and continuous decrease in atomic and ionic radii. This phenomenon is termed the Lanthanoid Contraction.
As atomic number increases across the series, a new electron is added to the deep, inner $4f$ subshell. The shape of f-orbitals is highly diffused, meaning they offer extremely poor shielding of the outer electrons from the growing nucleus. The increasing nuclear charge (+1 per element) easily overpowers this poor shielding, pulling the entire electron cloud steadily inward.
Because the 14 lanthanoids cause a massive shrinkage in size, the expected size increase from the 4d to the 5d series is perfectly cancelled out.
Radius of Zirconium ($Zr, 4d$) $\approx$ Hafnium ($Hf, 5d$)
This makes them almost identical in chemical properties and extremely difficult to separate.
As the size of the $Ln^{3+}$ ion decreases from $La^{3+}$ to $Lu^{3+}$, its charge density increases. According to Fajans' Rules, the covalent character of the $Ln-OH$ bond increases.
$La(OH)_3$ (Most Basic) $\rightarrow$ $Lu(OH)_3$ (Least Basic)
4. Color, Paramagnetism & Mischmetall
Most lanthanoid ions are colored in both solid and aqueous states. The color arises specifically from f-f transitions (electrons jumping between different f-orbital energy levels). Ions with $f^0$ (like $La^{3+}$) and $f^{14}$ (like $Lu^{3+}$) are colorless.
Paramagnetism is governed by the number of unpaired f-electrons. It reaches a maximum at Neodymium ($Nd$) and drops to zero at $f^0$ and $f^{14}$.
A famous commercial alloy containing ~95% Lanthanoid metals, ~5% Iron, and traces of S, C, Ca, and Al. It is mixed with Magnesium to produce bullets, shells, and lighter flints.
5. Actinoids vs Lanthanoids
Actinoids are the 5f series. They are all radioactive (unlike lanthanoids, where only Promethium ($Pm$) is radioactive). Because the 5f, 6d, and 7s energy levels are extremely close to each other, actinoids exhibit a much larger number of oxidation states (up to +7 for Np and Pu) compared to lanthanoids.
NEET Grand Test: f-Block Elements
15 High-Yield Questions testing contractions, oxidizing/reducing anomalies, and electronic configurations.
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