Introduction:

We have studied the main features of the chemistry of transition elements of the d-block. We learnt that in addition to the usual vertical relationship, the transition elements exhibit a horizontal similarity in their physical and chemical properties. We are now going to study the salient characteristics of the chemistry of transition elements of the f-block. Due to filling of electrons in the f-orbitals of an inner shell, these elements are as well known as inner-transition elements. The f-block elements constitute two series of elements - the lanthanide series and the actinide series. We will notice that in comparison to the elements of d-block transition series, the members of lanthanide series resemble one other much more closely. They have usually one common stable oxidation state and take place altogether in the similar ores in nature. Due to the similarity in their chemical properties their separation from one other is extremely difficult. Thus, special methods of solvent extraction and ion exchange are used for their separation. On the other hand, the chemistry of actinides is quite complex as they show more than one oxidation state and their radioactivity crafts problems in the study of their properties.

Though, the actinides do show several similarities with one other and by their lanthanide congeners in a specific oxidation state.

General Characteristics of Inner-Transition Elements:

We are familiar that the fourteen elements from cerium (Z = 58) to lutetium (Z = 71) that follow lanthanum (Z = 57) in the periodic table, are known as lanthanides, lanthanoids or lanthanons. It will be noted that some authors comprise lanthanum as well in lanthanides; however there is no general agreement on it. These elements are characterized through successive filling of 4f orbitals in their atoms. These elements all along with the lanthanum and yttrium were originally known as rare earth elements or simply rare earths. The term 'earth' was employed as they take place as oxides, which in early usage meant earth, and the term rare was used due to the great difficulty in their separation from one other. Or else, these are not specifically rare in the earth's crust. For illustration, lanthanum, cerium and neodymium are more rich than lead. Even the scarcest of them, thulium, is as rich as bismuth and richer than arsenic, cadmium, mercury and selenium, none of which is usually considered rare.

The 14 elements from thorium (Z = 90) to lawrencium (Z = 103) following actinium in the periodic table are recognized as actinides, actinoids or actinons. They are analogous to the lanthanides and outcome from the filling of 5f orbitals as the lanthanides result from the filling of 4f orbitals. Prior to 1940, only the naturally occurring actinides, that is, thorium, protactinium and uranium were acknowledged. The remaining actinides have been generated artificially since then and are collectively termed as transuranium elements. 

Electronic Configuration and Position in Periodic Table:

The outstanding characteristic of the lanthanide and actinide elements is of huge similarity in the physical and chemical properties which they display in each series.  The reason for this exclusive behavior of these elements lies in their electronic configuration.

We are familiar that lanthanum, the element preceding the lanthanides in the periodic table, consists of the electronic configuration [Xe]5d¹⁶s². Similar to lanthanum, the lanthanides as well show the stable oxidation state of +3. It is, thus, expected that in these elements the successive electrons will be filled in the 4f orbitals, thus the elements might have the electronic configuration from [Xe]4f¹⁵d¹⁶s² to [Xe] 4f¹⁴5d¹⁶s². The real ground state electronic configurations of lanthanide elements have been found out via atomic spectroscopy and are represented in the table shown below. We can notice from the above table that there is an electron in 5d orbital only in Ce, Gd and Lu, in all other elements this electron is shifted to the 4f orbital. This kind of shuttling of electrons can be understood in terms of the comparable energies of 4f and 5d orbitals. Whether there is an electron in the 5d orbital or not, is of little significance as the lanthanides generally form ionic compounds in +3 oxidation state and the electronic configuration of M³⁺ ions differs in a regular manner from [Xe]4f¹ for Ce³⁺ to [Xe]4f¹⁴ for Lu³⁺, as illustrated in the table shown.

Table: Properties of lanthanum and lanthanides

The ground state electronic configuration of actinium, [Rn]6d¹7s² is identical to that of lanthanum and certainly the two elements possess alike chemical properties. The electronic configurations of the elements that follow actinium are not acknowledged precisely; these are less certain than those of the lanthanide elements. The difference in energy between 5f and 6d orbitals in the starting of the actinide series is less than that between the 4f and 5d orbitals for the lanthanides. Thus, both 5f and 6d orbitals are comprised in accommodating successive electrons. Therefore the filling of 5f orbitals in actinides (Table shown below) is not quite so regular as the filling of 4f orbitals in case of the lanthanides. Later, though, the 5f orbitals become more stable, that is, by the time plutonium and following members of the series are reached, the 5f orbitals seem evidently to be of lower energy than the 6d orbitals, and therefore the electrons preferably fill the former.

Table: Properties of actinium and actinides

Atomic Radius:

We have studied that the atomic size decreases by increase in atomic number all along any period in the long form of the periodic table because of the increase in effective nuclear charge. Though, the decrease in atomic radius is small if the difference in electronic configuration from one element to the subsequent is that of an additional inner electron. This is due to the reason that the additional inner electron screens the size-determining outer electrons from the nucleus much better than an additional outer electron. For illustration, decrease in the covalent radius from Sc to Zn, that is, across ten elements of the 3d transition series, is 19 pm. This decrease is nearly one-third of the decrease in the covalent radius of the seven elements of s and p blocks of the period 3.

The rate of decrease in atomic radius all along  the lanthanide series and as well all along the actinide series is even less than that in the transition series, as the difference in the electronic configurations of such elements is in the number of electrons in the ante-penultimate (last however two) shell of electrons. However the additive effect of decrease in the atomic radius across the fourteen elements of lanthanide series is fairly substantial. This decrease in atomic radius across the lanthanide series is termed as lanthanide contraction. Likewise, there is an actinide contraction across the actinide series. As an outcome of lanthanide contraction, the normal increase in size from Sc→Y→La disappears after the lanthanides, and pairs of elements like Zr and Hf, Nb and Ta, Mo and W and so on, possess almost similar sizes (Table shown below). The properties of these elements, thus, are very similar. The similarities in properties in these pairs make their separation extremely difficult. Therefore, due to the lanthanide contraction, the elements of 5d and 4d transition series resemble each other much more closely than do the elements of the 4d and 3d series.

Table: Atomic (covalent) radii of the elements preceding and following lanthanides in pm

Oxidation States:

The sum of first three ionization energies of the lanthanides is rather low, therefore the elements are highly electropositive. They readily form M³⁺ for the lanthanides, actinium and trans-americium (Cm to Lr) elements the tripositive oxidation state is the most stable in each and every case. This is believed that in forming tripositive lanthanide or actinide ions, the ns² (n = 6 or 7) electrons are lost all along with the (n-1) d¹ electron. In the absence of (n-1) d¹ electron one of the electrons present in the (n - 2) f orbitals is lost.

Alongside the +3 state, some of the lanthanides and actinides exhibit other oxidation states as well. In such cases there is some proof that ions with f⁰ (example: La³⁺, Ce⁴⁺, Ac³⁺, Th⁴⁺, Pa⁵⁺, U⁶⁺); f⁷(example: Eu²⁺, Gd³⁺, Tb⁴⁺, Cm³⁺, Bk⁴⁺) and f₁₄ (example: Yb²⁺, Lu³⁺) configurations show greater stability. Though, Pr⁴⁺ (4f¹), Nd⁴⁺ (4f²), Sm²⁺ (4f⁶), Tm²⁺ (4f¹³) and so on. By non-f^o, non-f² and non-f¹⁴ electronic configurations as well exist. This reminds us that there might be other factors as well such as ionization energies and sublimation energies of the metals and lattice energies, and so on, which are responsible for the stability of such oxidation states. The known oxidation states of actinium and the actinides are given in the table shown below in which numbers in bold point out the most stable oxidation state in the aqueous solution. We can observe from the table which almost all the actinides show at least two stable oxidation states and oxidation states higher than +3 are simply accessible in the early actinides. For thorium, protactinium and uranium the highest accessible oxidation state is the most stable one as well in aqueous solution. This might be as 5f orbitals extend further from the nucleus than the 4f orbitals and 5f electrons are more efficiently shielded from the nuclear charge than are the 4f electrons of the corresponding lanthanides. As the 5f electrons are less firmly held, they are all available for bonding in the early actinides. Though, as the later actinides are approached, the build-up of nuclear charge causes-contraction of the 5f orbitals in such a way that the metal-ligands overlap reduces and the +3 state becomes predominant. Interestingly, the +2 state that is achievable in case of mendelevium and nobelium is more stable than Eu²⁺.

Table: Oxidation states of actinium and actinides

Colour of Ions:

The ions of lanthanides and actinides are colored in the solid state and also in aqueous solution, as is the case with the ions of transition metals. We know that the colours of transition metal ions occur due to absorption of light because of d-d electronic transitions. As there are no electrons in the d-orbitals, the colours of lanthanide and actinide ions occur because of electronic transitions in the 4f and 5f orbitals. The colours of hydrated lanthanide and actinide ions are illustrated in the first and second table, correspondingly.

Electrode Potentials:

The standard electrode potentials of lanthanides for the half-reaction,

Ln³⁺ (aq) + 3e → Ln (s) 

are illustrated in the first table. The electrode potentials are extremely low. Thus, these elements are highly electropositive and reactive metals. The electrode potential rises from Ce to Lu that is consistent by the slight decrease in the ionic radius because of lanthanide contraction. The electrode potentials of the actinide elements as well are quite low (second table illustrated above). Thus, the actinides as well are highly electropositive and reactive metals.

Complexation Behavior:

The ions of lanthanide and actinide elements encompass a strong tendency to form complexes by a variety of oxygen and nitrogen donor ligands. Perhaps due to their comparatively higher charge to size ratio, the actinide ions encompass a greater tendency to form complexes than the lanthanides. As well, due to the existence of a large number of oxidation states, the Complexation behavior of actinides is more varied. The lanthanide and actinide ions form the most stable complexes by chelating ligands like oxalic acid, citric acid, tartaric acid, nitric acid, ethylenediamine tetra acetic acid (EDTA) and β-diketones. In such complexes the metal ions encompass very high coordination numbers. For illustration, the coordination number of the metal ion in [Th(acac)₄], [Ce(NO₃)₄^-(OPPh₃)₂] and [Ce(NO₃)₆]^2- is 8, 10, 12, correspondingly. In such complexes, the acetyl acetonate (acac) and the nitrate ligands are acting as the bidentate ligands occupying two coordination sites around the metal ion. Such metal ions form water soluble complexes by citric acid, tartaric acid and EDTA. The formation of water soluble complexes by these ligands facilitates separation of the metal ions through ion exchange chromatography.

Magnetic Properties:

We are familiar that paramagnetism is related by the presence of unpaired electrons in a substance. The lanthanide and actinide ions, other than f^o kind (example: La³⁺, Cc⁴⁺. Ac³⁺, Th⁴⁺, Pa⁵⁺, U⁶⁺) and f¹⁴ kind (example: Yb²⁺, Lu³⁺, Lr³⁺) are all paramagnetic, as each of the seven f  orbitals characterizing inner-transition metal species (lanthanide and actinide) should have a single electron before any pairing can occur (that is, Hund's rule).

We have as well studied that in case of transition elements; the contribution of orbital motion of electrons to paramagnetism is negligible and can be ignored. The magnetic moments of transition metal ions can be described in terms of unpaired electrons present in the d-orbitals. However the magnetic moments of only such lanthanide ions, which encompass f^o, f⁷ and f¹⁴ configuration agree by the spin only value. In all the other cases, the magnetic moment values are higher than those computed on the basis of spin only formula. Though, these can be described by taking orbital contribution to magnetic moment as well into account. In lanthanide ions, the 4f orbitals are comparatively better shielded from the surroundings via the overlying 5s and 5p orbitals than the d-orbitals in the transition metal ions. Thus, the contribution of orbital motion to the paramagnetism is not quenched.

However actinides exhibit a variation in the magnetic properties identical to that of the lanthanides, the magnetic properties of actinide ions are more complex than those of the lanthanide ions. This in part occurs from (a) the fact that 5f electrons are closer to the surface of the atom and is simply affected by the chemical environment; however not to the similar extent as do the d electrons, and (b) the less sharply stated distinctions between the 5f and 6d electrons as compared by 4f and 5d electrons. From the discussion above it is obvious that the magnetic moments of the f-block (that is, inner transition) metal ions should be computed taking into account both the spin and orbital contributions.

Chemical Properties:

The lanthanides are generally silvery-white, highly electropositive and reactive metals. They all react slowly by cold water and fast on heating to release hydrogen:

2M + 6H₂O → 2M(OH)₃ + 3H₂

The hydroxides are basically ionic and basic. They are less basic than Ca(OH)₂ however more basic than the amphoteric Al(OH)₃. The base strength reduces from Ce(OH)₃  to Lu(OH)₃ as the ionic radius reduces from Ce³⁺ to Lu³⁺. The lanthanide metals dissolve in dilute acids, even in the cold, to release hydrogen gas:  

2Ln + 6HCI → 2LnCl₃ + 3H₂

The metals tarnish readily in air making an oxide coating. On heating in oxygen, they burn simply to provide M₂O₃, apart from for cerium that forms CeO₂. The oxides are ionic and basic; the base strength reduces as the ionic radius decreases.

4Ln + 3O₂ → 2Ln₂O₃

If heated in halogens, the lanthanides bum generating LnX₃ that can as well be made via heating the oxides by the suitable ammonium halide:

2Ln + 3X₂ → 2LnX₃ 

Ln₂O₃ + 6NH₄X → 2LnX₃ + 6NH₃ + 3H₂O

Cerium with fluorine makes CeF₄: 

Ce + 2F₂ → CeF₄

The metals react exothermically with hydrogen, although heating to 600-700 K is frequently ^8 needed to initiate the reactions. Their hydrides are non-stoichiometric compounds having ideal formulae, MH₂ and MH₃. The hydrides are extraordinarily stable to heat up to 1200 K. The hydrides react by water liberating hydrogen gas:

MH₃ + 3H₂O → M(OH)₃ + 3H₂

On heating, the lanthanides react by boron giving borides of the kind MB₄ and MB₆, by carbon giving carbides M₂C₃ and MC₂ and with nitrogen giving nitrides MN. A broad variety of their oxosalts, such as carbonates, sulphates, nitrates, phosphates, oxalates and so on, are known.

All the actinides are not stable with respect to the radioactive disintegration, although the half-lives of the richest isotopes of thorium and uranium are so long that for numerous purposes their radioactivity can be neglected. Similar to lanthanides, actinides are as well electropositive and reactive metals. They react by water, oxygen, hydrogen, halogens and acids. Their hydrides are non-stoichiometric having ideal formulae MH₂ and MH₃. The metals as well react with most of the non-metals particularly whenever heated.

Occurrence, Extraction and Uses:

Each and every lanthanide and actinide elements are highly reactive metals, thus, none of them takes place in the Free State in nature. Furthermore, all the actinide elements are radioactive; therefore most of them don't take place naturally and have been made artificially since 1940.

Occurrence:

Apart from the promethium which is unstable and is found in traces in uranium, ores, all the lanthanides usually take place altogether. However a large number of minerals are known to have lanthanides, only three of them, namely, monazite, bastnaesite and xenotime are of commercial significance. Monazite and xenotime are the mixture of phosphates of thorium, lanthanum and lanthanides. Monazite is broadly however sparsely distributed in most of the rocks, but due to its high density and inertness, it is concentrated via weathering into sands on beaches and river beds. Deposits of monazite take place in Southern India, South Africa and Brazil. Bastnaesite is the mixture of fluoride carbonates, LnFCO₃, of lanthanum and lanthanides. Both monazite and bastnaesite are richer in the lighter lanthanides, that is, the cerium earths, however with the difference that monazite as well contains upto 30% ThO₂, which is absent in the bastnaesite. On the other hand xenotime is a priceless source of the heavier rare earths.

Each and every known isotope of the actinide elements is radioactive and their half-lives are such that only ²³²Th, ²³⁵ U, ²³⁸U and possibly ²⁴⁴Pu have survived throughout the very period of their existence. Only thorium and uranium are found in nature in amounts adequate for practical extraction. Thorium comprises 8.1 x 10^-4 % of the earth's crust and it is nearly as rich as boron. As illustrated earlier, monazite is the most significant source of thorium. Uranium constitutes 2.3 X 10^-4 % the earth's crust and it is slightly richer than tin. Pitchblende or uraninite (U₃O₈) and carnotite, K₂(UO₂)₂(VO₄)^2- 3H₂O, are two significant ores of uranium.

Extraction:

As all the lanthanides take place altogether in nature, their extraction comprises two main steps: (a) separation from one other and (ii) reduction of their compounds to metals. As the lanthanides are all usually trivalent and are almost alike in size, their chemical properties are nearly similar. Thus, the separation of lanthanides from one another is an extremely complicated task, nearly as difficult as the separation of isotopes. Only cerium and europium can be separated from the remaining lanthanides via using conventional chemical methods as stabilities of Ce⁴⁺ and Eu²⁺ in aqueous solution. Cerium can be separated from the mixture of lanthanides via oxidising Ce³⁺ to Ce⁴⁺ with permanganate or bromate or hypochlorite in the alkaline medium and afterward precipitating it as CeO₂. Europium can be reduced to Eu²⁺ either via electrolytic reduction by a mercury cathode or by employing zinc amalgam. This is then precipitated from the solution as EuSO₄.

Previously the lanthanides are used to be separated from each other by selective precipitation or by fractional crystallization. By a limited amount of a precipitating agent, the substance that is least soluble is precipitated first. For illustration, if a base is added to a solution of lanthanide nitrates, the least soluble Lu(OH)₃  is precipitated first and the most soluble La(OH)₃ last. As merely a partial separation is effected, the precipitate is re-dissolved and the method is repeated many times.

The solubility of double salts of lanthanides like 2Ln(NO₃)₃.3Mg(NO₃)₂.24H₂O and Ln₂(SO₄)₃ Na₂SO₄.xH₂O increases from La to Lu. Thus, the lanthanides could be separated from one other via fractional crystallization of such salts. As these methods need to be repeated many times, these are very tedious and not very proficient. Though, the individual elements can now be separated by much less difficulty on a large scale via employing more efficient methods of solvent extraction and ion exchange chromatography.

The distribution coefficients of salts of lanthanide elements between water and organic solvents are slightly dissimilar. Thus, the individual elements are selectively extracted from the aqueous solutions of their salts to an organic solvent. This method of separation is termed as solvent extraction. Tributyl phosphate is a very good solvent for this method. The solubility of lanthanides in +3 oxidation state in Tributyl phosphate rises with the atomic number. Separation is performed by employing a continuous counter-current process in which the aqueous solution of lanthanide nitrates and the solvent are passed via a column continuously in the opposite directions. This method is much less tedious than performing some crystallization.

The procedure of ion exchange chromatography is the most significant, rapid and efficient method for the separation and purification of the lanthanons. In this method, a solution of lanthanide ions is run down a column of a synthetic ion exchange resin. Ion exchange resins are organic polymers comprising of functional groups like -COOH, -SO₃H or -OH. In such resins, hydrogen ions are mobile and can be exchanged by other cations. Therefore, the lanthanide ions substitute the H+ ions and get bound to the resin:

Ln³⁺ + 3R - SO₃H → Ln(SO₃R)₃ + 3H⁺

After the H+ ions have passed via the column, a solution of a complexing agent like citric acid, α-hydroxyisobutyric acid or EDTA at the suitable pH is passed via the column to elute, that, to wash off the metal ions in a selective way:

Ln(O₃SR)₃ + (NH₄)₃EDTAH → Ln(EDTAH) + 3NH₄O₃SR

As the 'EDTA' solution flows down the column, the lanthanide ions come off the resin and form a complex by EDTA and then go back on the resin a little lower down the column. This method is repeated numerous times as the metal ions steadily travel down the column. The smaller lanthanide ions such as Lu³⁺ form stronger complexes by EDTA than the larger ions such as La³⁺. Therefore, the smaller and heavier ions spend more time in solution and less time on the column. Thus, the heavier ions are eluted from the column first and the lighter ones the last. By employing appropriate conditions, all the individual elements can be separated. The eluates are then treated by an oxalate solution to precipitate the lanthanides as oxalates which are then ignited to obtain the oxides:

2Ln(EDTAH) +3(NH₄)₂(C₂O₄)₃ → Ln₂(C₂O₄)₃ + 2(NH₄)₃EDTAH

2Ln(C₂O₄)₃ → Ln₂O₃ + 3CO + 3CO₂

Samarium, europium and ytterbium are made by the reduction of oxides by La at high temperatures:

2LnO₃ + 2La → Ln₂O₃ + 2Ln, Ln = Sm and Eu

Other lanthanides are obtained via the reaction of LnCl₃ or LnF₃ by Ca metal at 1300 K. LnCl₃ or LnF₃ are made up by heating Ln₂O₃ by means of suitable ammonium halide:

Ln2O3 + 6NH4X → LnX3 - 6NH3 + 3H2O

2LnX₃ + 3Ca → 2Ln + 3CaX₂

We are familiar that actinium and all the actinides are radioactive. Of these just thorium and uranium are extracted from the ores, all others are made artificially via nuclear reactions. The main ores of thorium and uranium are monazite and pitchblende, correspondingly. For extraction of thorium, monazite is dissolved in concentrated sulphuric acid. By correctly adjusting the pH of this solution, a precipitate of ThO₂ is obtained. The impure ThO₂ is purified via dissolving it in hydrochloric acid and then extracting ThCl₄ via trihutylphosphate. From this solution ThO₂ is re-precipitated via adjusting the pH. Purified ThO₂ is transformed to anhydrous ThF₄ or ThCl₄ by the action of HF or CCl₄ at 900K. Thorium metal is then made by the reduction of ThF₄ or ThCl₄ with calcium:

ThX₄ + 2Ca → Th + 2CaX₂ 

Uranium is mainly extracted from the pitchblende. The concentrated ore (pitchblende, U₃O₈) is washed and then fused by sodium carbonate and sodium nitrate. The fused mass is treated with sulphuric acid that extracts uranyl sulphate, UO₂SO₄. Addition of sodium carbonate solution in surplus to the above solution eliminates all the heavy metals as carbonates. Uranium goes in solution as sodium uranyl carbonate Na₄[(UO₂)(CO₃)₃]. Addition of dilute H₂SO₄  to the Uranyl carbonate solution precipitates uranium as sodium diuranate, Na₂U₂O₇, which on treatment by concentrated solution of (NH₄)₂CO₃ passes to solution as ammonium uranyl carbonate, (NH₄)₄[UO₂(CO₃)₃]. Concentration of this solution provides pure U₃O₈. Reduction of U₃O₈ by aluminium powder generates uranium metal. All such steps involved in the extraction of uranium from pitchblende are summarized below.

Fig: Extraction of uranium from pitchblende

Uses:

Lanthanides and most of their complexes have received broad industrial applications. For illustration, europium derivatives are employed as phosphors in TV screen; samarium-cobalt alloys are employed for making magnets, Pr₂O₃ and Nd₂O₃ are employed for making welder's goggles, yttrium-aluminium garnets (YAG) are employed both in electronic equipment and as synthetic gems. Different mixed oxides are employed as catalysts in cracking of petroleum. Cerium in the +4 oxidation state is employed as an oxidising agent in the quantitative analysis. Thorium nitrate has been employed for more than a century in gas mantles. Till the year 1940, the only industrial application of uranium was as a coloring material in the manufacture of yellow glass. At present, the principal make use of thorium and uranium is as a nuclear fuel.

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