Lanthanoid Contraction

CHEInner Transition Elements

Lanthanoid contraction is the steady decrease in atomic and ionic size from $La$ to $Lu$ as the $4f$ subshell fills. It explains the close similarity of $4d$ and $5d$ congeners and the gradation in properties down the series.

Filling of the 4f subshell across the series

La:[Xe]5d^1 6s^2 \;\rightarrow\; Ce:[Xe]4f^1 5d^1 6s^2 \;\rightarrow\; Lu:[Xe]4f^{14}5d^1 6s^2

the

4f

electrons shield the nucleus poorly, so

Z_{\text{eff}}

on outer shells rises from

La

Lu

Definition: the regular, small decrease in atomic and $M^{3+}$ ionic radii across the lanthanoids from $La$ to $Lu$ .
Cause: poor, imperfect shielding by $4f$ electrons, so as nuclear charge increases the effective nuclear charge felt by outer electrons rises steadily and pulls the shells inward.
Consequence: second ( $4d$ ) and third ( $5d$ ) transition series congeners have almost equal radii — $Zr$ and $Hf$ (also $Nb/Ta$ , $Mo/W$ ) are nearly identical in size and so very similar chemically and hard to separate.
Consequence: basicity of $Ln(OH)_3$ decreases from $La(OH)_3$ to $Lu(OH)_3$ , because the shrinking $Ln^{3+}$ holds $OH^-$ more tightly (rising charge/size ratio increases covalent character).
Consequence: the lanthanoids are so alike in size that they occur together and are difficult to separate by ordinary methods (ion-exchange or solvent extraction is needed).
Lanthanoids vs actinoids: actinoids show a wider range of oxidation states and are all radioactive, whereas lanthanoids are mostly $+3$ and only promethium is radioactive.
The total contraction is small (about $0.18$ Å overall) but cumulative — the $4f$ subshell is buried deep inside, so each successive step shrinks the radius only slightly.
Because $4f$ orbitals lie deep inside, the lanthanoids show very similar chemistry (almost all $+3$ ), and their compounds differ only gradually in size-dependent properties.
The contraction is the reason the third transition series elements ( $Hf$ , $Ta$ , $W$ , etc.) are much denser than the second series, even though they are one period lower.
A similar but smaller effect, the actinoid contraction, occurs across the $5f$ series for the same shielding reason.

Where the marks go

Attributing the contraction to $5d$ or $6s$ electrons; it is the poor shielding by the inner $4f$ electrons that causes it.
Saying basicity of $Ln(OH)_3$ increases across the series; it decreases from $La(OH)_3$ to $Lu(OH)_3$ as the ion shrinks.
Claiming $Hf$ is much larger than $Zr$ because it is a period below; the lanthanoid contraction makes their radii nearly equal.
Confusing lanthanoid contraction (size decrease across the $4f$ series) with the general atomic-radius trend across a $d$ -block period.
Stating all lanthanoids are radioactive; only promethium ( $Pm$ ) is, whereas all actinoids are radioactive.

How the board asks it

Give reasonscause: poor shielding by $4f$ electrons
Account for the steady decrease in atomic and ionic radii of the lanthanoids from $La$ to $Lu$ .
Give reasonsconsequence: nearly equal radii of $4d$ and $5d$ congeners
Give reasons: $Zr$ and $Hf$ have almost identical atomic radii and are very difficult to separate.
Define / statedefinition and cause
What is meant by lanthanoid contraction? Name the subshell whose imperfect shielding is responsible for it.
Give reasonsconsequence: basicity of $Ln(OH)_3$ decreases
Account for the fact that the basic strength of the hydroxides decreases from $La(OH)_3$ to $Lu(OH)_3$ across the series.
Distinguishlanthanoids vs actinoids
Give two points of difference between the lanthanoids and the actinoids with reference to their oxidation states and radioactivity.
Assertion–Reasonconsequence: density of third transition series
Assertion: The third transition series elements such as $Hf$ and $W$ are much denser than their second-series congeners. Reason: Lanthanoid contraction keeps their atomic radii nearly equal while atomic mass increases. State whether both are correct and whether the Reason correctly explains the Assertion.

Practise this topic

PreviousPotassium Permanganate: Preparation & Reactions NextComplex Ions, Oxoanions & Mixed

Written for Sublevo. Question text quoted anywhere in these notes is the Council’s and carries its year and paper; the board’s own diagrams are not reproduced.