Lanthanoid 

General Introduction of Lanthanides

• The term lanthanide was introduced by Victor Goldschmidt in 1925.

• The lanthanide series consists of 15 elements with atomic numbers ranging from 57 (Lanthanum) to 71 (Lutetium), usually placed separately at the bottom of the periodic table.

• These elements are called lanthanides because of their close similarity in chemical properties to Lanthanum (La). They are collectively represented by the general symbol Ln.

• Lanthanides and actinides are known as inner transition metals, whereas Scandium (Sc), Yttrium (Y), Lanthanum (La), and lanthanides together are commonly referred to as rare earth metals since these elements were historically found as oxides (earth) and were relatively scarce.
  According to the International Union of Pure and Applied Chemistry (IUPAC) Red Book (1985), the term "lanthanoid" is preferred over "lanthanide" because the suffix "-ide" generally denotes a negatively charged ion.
  However, due to its widespread usage, the term lanthanide remains acceptable in modern literature.

• Electronic Configuration

• They have an electronic configuration of [Xe] 4f1-14 5d0-1 6s2 of the 14 lanthanides. 

• Note: The energies of 4f and 5d electrons are almost close to each other, so the 5d orbital remains vacant, and the electrons enter the 4f orbital.

Oxidation States

• They show +2, +3, and +4 oxidation states. But ionization beyond the M3+ ion is energetically possible, and this leads to a most stable characteristic +3 oxidation state.

Oxidation States of Lanthanides

• The most common oxidation state of lanthanides is +3.

• However, some lanthanides also show +2 and +4 oxidation states under specific conditions.

Important Points:

• +2 Oxidation State is shown by:
  Sm²⁺, Eu²⁺, Yb²⁺, Nd²⁺, Tm²⁺
  (Mainly stable in aqueous solution or with halide ions like iodide or bromide)

• +4 Oxidation State is shown by:
  Ce⁴⁺, Pr⁴⁺, Nd⁴⁺, Tb⁴⁺, Dy⁴⁺
  (Stabilized mostly with fluoride or oxide ions)

Oxidation State                     Stability Reason                                              Example

+2                                      Half-filled or fully-filled 4f orbitals                         Eu²⁺ (4f⁷), Yb²⁺ (4f¹⁴)

+4                                      Empty or half-filled 4f orbitals                              Ce⁴⁺ (4f⁰), Tb⁴⁺ (4f⁷)

Reducing and Oxidizing Nature:

• Sm²⁺ → Easily loses an electron → Strong reducing agent
  Sm²⁺→Sm³⁺+e
  Ce⁴⁺ → Easily gains an electron → Strong oxidizing agent
  Ce⁴⁺+e−→Ce³⁺

Ionic Radii and Lanthanide Contraction

• In general, atomic and ionic radii increase down a group in the periodic table. However, in the lanthanide series (Ce to Lu), a unique trend is observed:
  → Ionic and atomic radii decrease gradually across the series, from Cerium (Ce) to Lutetium (Lu).

• This consistent decrease in size, despite the addition of electrons, is called lanthanide contraction.
  
  

Key Observations:

• Ionic radii of Ln³⁺ ions decrease smoothly across the series.

• Atomic radii, which are measured in metals (metallic radii), also decrease but less regularly:
  For example:  Ce: 165 pm; Lu: 156 pm; Overall decrease = 9 pm

• However, Europium (Eu) and Ytterbium (Yb) show anomalously high atomic radii:

  • Eu: 185 pm, Yb: 170 pm

  • This is due to their electronic configurations and lower metallic bonding strength.

In the case of Europium (Eu) and Ytterbium (Yb), only the two 6s electrons participate in metallic bonding, as their 4f subshells are fully stable and do not contribute to bonding.

 In contrast, other lanthanides typically have three electrons (including 5d or 4f) involved in metallic bonding.

 As a result, the metallic bonding in Eu and Yb is weaker, leading to larger atomic volumes and, consequently, larger atomic radii compared to other lanthanides.

 Cause of Lanthanide Contraction

• As we move from Cerium (Ce) to Lutetium (Lu) in the lanthanide series, one electron is added to the inner 4f-orbitals at each step.
  
  

• However, 4f-electrons provide very poor shielding because they are deeply buried and have a diffused shape.
  
  

• At the same time, the nuclear charge (number of protons) increases by one at each step, increasing the attraction between the nucleus and outer electrons.
  
  

• Due to poor shielding by 4f-electrons, this increased nuclear attraction pulls the outer electrons closer to the nucleus, causing a gradual decrease in atomic and ionic size.
  Conclusion:

 Lanthanide contraction occurs in the 4f-series elements due to:
→ Poor shielding effect of 4f-electrons
→ Gradual increase in nuclear charge

Consequences of Lanthanide Contraction

1. Basic Character of Lanthanide Hydroxides (Ln(OH)₃):

• Due to lanthanide contraction, the covalent character of Ln³⁺–OH⁻ bond increases from La(OH)₃ to Lu(OH)₃.
  
  

• Hence, the basic strength of hydroxides decreases across the series.
  
  

→ La(OH)₃ = Most basic
→ Lu(OH)₃ = Least basic

(As per Fajans' Rule – smaller cations form more covalent bonds)

2. Similarity in Atomic Radii of 2nd and 3rd Transition Series:

• Due to lanthanide contraction, the atomic radii of elements in the 2nd (4d) and 3rd (5d) transition series become nearly equal.
  
  

• This happens because the inclusion of 14 lanthanides between La (Z = 57) and Hf (Z = 72) prevents the expected increase in atomic size.
  
  

Occurrence and Isolation:

The lanthanides, comprising elements with atomic numbers 57 to 71, are a series of f-block elements often referred to as rare earth elements. Despite this label, most lanthanides are relatively abundant in the Earth’s crust. Their “rarity” stems more from the difficulty in their separation and purification due to similar ionic radii and chemical properties rather than their actual scarcity.

 Lanthanides primarily occur in nature as oxides or phosphates in minerals such as monazite, bastnäsite, and xenotime. These minerals are commonly found in placer deposits, and extraction typically involves complex chemical treatments, including ion exchange and solvent extraction techniques, to isolate individual lanthanides.

 Among the lanthanides, Thulium (Tm) is considered the rarest naturally occurring member. However, it is still more abundant than some non-lanthanide elements. For instance, thulium occurs in the Earth's crust at approximately 4.5 × 10⁻⁵ % by mass, which is higher than that of silver, at about 0.79 × 10⁻⁵ % by mass.

• Promethium (Pm), with atomic number 61, is the only synthetic radioactive element.

 Oddo-Harkins Rule: 

The Oddo-Harkins rule, proposed by Giuseppe Oddo and William Draper Harkins, is an empirical observation in geochemistry and cosmochemistry. It states:

• Elements with even atomic numbers are more abundant in the universe and Earth's crust than their adjacent odd-numbered elements.

• This trend is evident across a wide range of elements, particularly in the lanthanide and transition series.
  
  

Scientific Rationale:

• Even-numbered elements generally have greater nuclear stability due to paired protons and neutrons in their nuclei.
  
  

• As a result, they are more likely to form during stellar nucleosynthesis and are less prone to radioactive decay.
  
  

• Elements with even atomic numbers also tend to have a higher number of stable isotopes.
  
  

• In contrast, elements with odd atomic numbers rarely have more than two stable isotopes, making them less abundant.
  
  

Examples:

• Iron (Fe, Z = 26) is significantly more abundant than its neighbours, manganese (Mn, Z = 25) and cobalt (Co, Z = 27).
  
  

• In the lanthanide series, neodymium (Nd, Z = 60) is more abundant than praseodymium (Pr, Z = 59) and promethium (Pm, Z = 61) — the latter being radioactive and not found in nature.

Physical Properties of Lanthanides

Density

In the lanthanide (inner transition) series, the density generally increases with increasing atomic number. This trend is opposite to that of atomic radii — as atomic size decreases across the series, density increases. Lanthanides exhibit high densities ranging from 6.77 to 9.74 g cm⁻³.

 Melting and Boiling Points: 

Lanthanides possess relatively high melting and boiling points. However, no regular or uniform trend is observed across the series due to variations in their electronic structures and metallic bonding strength.

 Magnetic Properties

• Diamagnetic Ions:
  La³⁺ (4f⁰), Ce³⁺ (4f¹), and Lu³⁺ (4f¹⁴) exhibit diamagnetism due to either empty or fully-filled 4f orbitals, resulting in no unpaired electrons.
  Paramagnetic Ions:

• The remaining lanthanide ions are paramagnetic owing to the presence of unpaired electrons in their 4f orbitals, contributing to their magnetic behaviour.

µ(S+L) = √[4S(S+1)+L(L+1)]

Magnetic Moment (µ)

• The magnetic moment (µ) in Bohr Magneton (B.M.) is calculated by considering both spin (S) and orbital (L) contributions using the formula:
  S = Resultant spin quantum number
  L = Resultant orbital angular momentum quantum number

Special Case of First Transition Series

• In the first-row transition elements (3d-series), the contribution from the orbital motion (L) is generally suppressed (quenched) due to the strong interaction of the d-electrons with the crystal field (ligand field) created by surrounding ligands.
  
  

• Therefore, for these elements, the magnetic moment is effectively calculated using the spin-only formula:
  Where n = number of unpaired electrons.

 µS = √[4S(S+1) or µS = √[n(n+2)

Magnetic Properties of Lanthanide Ions

• The spin-only formula for the magnetic moment works well for La³⁺ (f⁰), Gd³⁺ (f⁷), and Lu³⁺ (f¹⁴), where orbital contributions are minimal or cancel out.

• For Gd³⁺ (f⁷ configuration), with 7 unpaired electrons:    μS=7(7+2)=63≈7.9 B.M.

• However, for most lanthanide ions, this simple relationship does not hold.
  The 4f electrons are deeply embedded and shielded by outer 5s and 5p orbitals, preventing crystal field quenching of orbital motion.
  
  

• As a result, orbital contributions are retained, and the total magnetic moment includes both spin (S) and orbital (L) contributions.

Spin-Orbit Coupling in Lanthanides

• In lanthanides, spin (S) and orbital (L) angular momenta couple to form a total angular momentum quantum number J.

• The value of J depends on the filling of the 4f Sub-shell:
  If the Sub-shell is less than half-filled:
  J=∣L−S∣

• If the Sub-shell is more than half-filled:
  J=L+S
  Additional Notes

• A similar behaviour is observed in 4d and 5d transition elements, where orbital contributions are more significant than in the 3d series.

• However, the magnetic properties of lanthanides are fundamentally different from those of transition metals due to the localized nature of 4f orbitals and strong spin-orbit coupling.
  
  

The magnetic moment μ is calculated in Bohr magneton by μ = g√J(J+1)

Where g = 3/2 + S(S+1)-L(L+1)/(2J(J+1)

Ceric Ammonium Sulphate and its analytical use

•Ceric sulfate is used in analytical chemistry for redox titration, often together with a redox indicator. A related compound is ceric ammonium sulfate. The solubility of Ce(IV) in methanesulfonic acid is approximately 10 times the value obtainable in acidic sulfate solutions.

•Formula: (NH4)4Ce(SO4)4·2H2O

•A crystallographic study shows that the compound contains the Ce2(SO4)88− anion, where the cerium atoms are 9 coordinated by oxygen atoms belonging to sulfate groups, in a distorted tricapped trigonal prism. The compound is thus sometimes formulated as (NH4)8[Ce2(SO4)8]·4H2O.

 Isolation of Lanthanides:

 All the metals (excluding Pm) can be obtained from monazite sand, a mixed phosphate (Ce, La, Nd, Pr, Th, Y . . .)PO4. Bastnӓsite, (Ce, La ...)CO3F, is a source of the lighter lanthanoids.

§Monazite sand is the best known and most important mineral of lanthanide elements which is essentially a mixture of orthophosphates, LnPO4 containing upto 12% thorium, the element of 5f series, small amounts of Zr, Fe and Ti as silicates, lanthanum and about 3% yttrium. Among lanthanides contained in monazite, the bulk is of Ce, Nd, Pr, and others occur in minute quantities.

•Monazite is treated with hot concentrated H2SO4 when thorium, lanthanum, and other elements dissolve as sulphates and are separated from insoluble material (impurities). On partial neutralisation by NH4OH, thorium is precipitated as ThO2. Then Na2SO4 is added to the solution.

•Lanthanum and light lanthanides are precipitated as sulphates leaving behind the heavy lanthanides in solution.

•To the precipitate obtained as above, is added hot conc. NaOH. The resulting hydroxides of light lanthanides are dried in air at 1000 °C to convert the hydroxides to oxides.

• The oxide mixture is treated with dil. HNO3/HCl. This brings CeO2 as precipitate and other lanthanides in solution.

Separation/Extraction of Individual Lanthanides:

Role of solubility in the separation:

To separate respective lanthanides the solubility of salts plays very important role.

Solubility depends upon on small difference between the lattice energy and the solvation energy and there is no obvious trend in the group.

Many lanthanides forms double salts, like Na2SO4Ln2(SO4)3.8H2O these salts are used to separate lanthanides from each other.

Separation of Lanthanides

Modern Ion-Exchange Method

Most rapid and most effective method for the isolation of individual lanthanide elements from the mixture.

§An aqueous solution of the mixture of lanthanide ions (Ln3+aq) is introduced into a column containing a synthetic cation exchange resin such as DOWAX-50 [abbreviated as HR (solid)].This  cation-exchange resin is  also called sulfonated polystyrene or  where its Na+ salt is used. When a solution containing Ln3+ ions is poured on to a resin column, the cations exchange with the H+ or Na+ ions.

Ln3+ (aq)  + 3H+ (resin) ↔ Ln3+ ( resin) +  3H+ (aq)

Ln3+ (aq )+ citrate ions → Ln-citrate complex

§The resin bound Ln3+  ions are now removed using a complexing agent such as EDTA4-.

§By using a long ion-exchange column, 99.9% pure components can be separated. Where Lu3+aq. ion is attached to the column with minimum firmness remaining at the bottom and Ce3+aq. ion with maximum firmness remaining at the top of the resin column.

Likewise if  the citrate solution (a mixture of citric acid and ammonium citrate) is used as the eluant, during elution process, NH4+ ions are attached to the resins replacing Ln3+aq. ions which form Ln-citrate complexes:

Ln (resin)3 + 3NH4+ → 3NH4- resin + Ln3+aq

Ln3+aq + citrate ions → Ln-citrate complex

The Ln3+aq cations with the largest size are, eluted first (heavier Ln3+aq ions)

Lighter Ln3+aq ions with smaller size are held at the top of the column.

Fractional Crystallization Method: This method is based on the difference in solubility of the salts such as nitrates, sulphates, oxalates, bromates, perchlorates, carbonates and double salts of lanthanide nitrates with magnesium nitrate which crystallize well and form crystals.

•Since, the solubility of these simple and double salts decreases from La to Lu, the salts of Lu will crystallize first followed by those of lighter members.

•A non-aqueous solvent, viz., diethyl ether has been used to separate Nd(NO3)3 and Pr(NO3)3 .

Fractional Precipitation Method: Based on the difference is solubility of the precipitate formed.

When NaOH is added to a solution of Ln(NO3)3, Lu-hydroxide being the weakest base and having the lowest solubility product is precipitated first while La-hydroxide which is the strongest base and has the highest solubility product is precipitated last. By dissolving the precipitate in HNO3 and reprecipitating the hydroxides a number of times, it is possible to get the complete separation of lanthanide elements.

Valency change Method

Based on the change of chemical properties by changing the oxidation state. The most important application of this method is made in the separation of cerium and europium elements from mixture of lanthanides.

•The mixture containing Ln3+ ions if treated with a strong oxidising agent such as alkaline KMnO4, only Ce3+ ion is oxidized to Ce4+ while other Ln3+ ions remain unaffected. To this solution alkali is added to precipitate Ce(OH)4 only, which can be filtered off from the solution.

•Eu2+ can be separated almost completely from Ln3+ ions from a solution by reducing it with zinc-amalgam and then precipitating as EuSO4 on adding H2SO4 which is insoluble in water and hence can be separated. The sulphates of other Ln3+ ions are soluble and remain in solution.

Complex Formation Method

Method is generally employed to separate heavier lanthanide elements from the lighter ones by taking the advantage of stronger complexing tendency of smaller cations with complexing agents.

•When EDTA is added to Ln3+ ion solution, lanthanides form strong complexes. If oxalate ions are added to the solution containing EDTA and Ln3+ ions, no precipitate of oxalates is obtained.

•On adding small amount of acid, the least stable complexes of lighter lanthanides are dissociated and precipitated as oxalates, but the heavier lanthanides remain in solution as EDTA complexes