f-block elements | inner transition elements | Lanthanides and actinides

f-block elements or inner transition elements

The elements that have incompletely filled (n-2) f orbitals in their ground state or in any of their oxidation states are called f-block elements or inner transition.

What are Lanthanides or Lanthanons or 4 f block elements? Explain its position in the periodic table

The lanthanides series is placed in a row below the periodic table. The lanthanides series consists of 14 elements which start from cerium (58) and end with Lutecium (71). Since the elements follow lanthanum in the periodic table they are called Lanthanides or Lanthanons. In these elements, the differentiating electron enters 4f orbital, hence these elements are also called 4f block elements.

Lanthanides have 6 shells and hence are placed in the 6^th period of the periodic table. They are all given only one position in the periodic table in Group 3 just below the yttrium.

A separate position for each lanthanide element was not possible as it would lead to sidewise expansion and destroy the symmetry of the main periodic table. Moreover, the elements with similar properties could not be grouped together.

What are Actinides or Actinons 5 f block elements? Give its position in the periodic table

The actinides series is placed in a row below the periodic table. The actinides series consists of 14 elements which start from thorium (90) and end with Lawrencium (103). Since the elements follow Actinium in the periodic table they are called Actinides or Actinons. In these elements, the differentiating electron enters a 5f orbital, hence these elements are also called 5f block elements.

Actinides have 7 shells and hence are placed in the 7^th period of the periodic table. They are all given only one position in the periodic table in Group 3 just below lanthanides.

A separate position for each Actinide element was not possible as it would lead to sidewise expansion and destroy the symmetry of the main periodic table. Moreover, the elements with similar properties could not be grouped together.

How are f-orbitals labeled based on the magnetic quantum number?

When Principal quantum number (n) = 4,

Then orbital angular momentum quantum number (l) = 0  to   n – 1

                                                                                      l = 0 to 4 – 1                                                                                         = 0 to 3

l = 0,   1,   2,  3

Therefore orbitals are = s,   p,   d,   f

Thus when, l = 3, the orbital is f.

Magnetic quantum number (m_l) = +l   to   –l

                                                m_l = +3   to   -3

                                                      = +3, +2, +1, 0, -1, -2, -3

When m_l = 0 ………………z³

           m_l = ±1………………xz², yz²

           m_l = ±2 ………………xyz, z(x² – y²)     

       m_l = ±3 ………………x(x² – 3y²), y(3y² – x²)

Explain the electronic configuration of f block elements, Lanthanides

i) Two electronic configurations for Lanthanides have been suggested, ideal electronic configuration and observed electronic configuration.

ii) In the idealized electronic configuration, lanthanum has an electronic configuration,     [Xe] 4f0 5d1 6s2. According to the Aufbau principle, once the 6s electrons are completely filled with electrons, the incoming electrons should occupy 5d orbitals. Once the 5d orbitals are filled up with electrons, filling up should start in the 4f orbitals. However, in practice, we observe that once the 5d orbital is occupied by one electron, the energy level of the 4f orbitals falls below that of the 5d, and subsequent electrons are thus added to the inner shielded 4f orbitals. Thus, the general idealized electronic configuration of these elements is represented as 4f^1-14 5d¹ 6s²

iii) In the case of observed electronic configuration, obtained from the spectroscopic and atomic beam resonance data, the 5d1 electron is transferred to the 4f level. The transfer of the 5d1 electron to the 4f orbital takes place in order to contribute stability to the lanthanide atoms whose pre-penultimate orbitals are not completely filled with electrons. The 4f orbitals in the observed electronic configuration have one electron more than the corresponding idealized electronic configuration.

There is a tendency for the 5d electron to drop to the 4f level because of the special stability of the half-filled and completely filled sub-level. Thus there is a tendency to achieve 4f⁷ and 4f¹⁴ arrangements as soon as possible and indeed these arrangements are achieved at Europium and Ytterbium before the mid-point and end of the series respectively. However, the shifting of 5d¹.

electron to 4f orbital is not observed in Gadolinium and Lutecium. This tendency of retaining 5d¹ electrons is due to enhanced stability associated with the half-filled and completely filled 4f-orbital in Gd and Lu respectively. 

iv) The difference between the ideal and observed electronic configurations is the presence or absence of 5d electron. The 4f and 5d levels are so close in energy that it is very difficult to locate the exact position of the electron.

Explain the electronic configuration of the f block elements, Actinides

i) Two electronic configurations for Actinides have been suggested, ideal electronic configuration and observed electronic configuration.

ii) In lanthanides immediately after Lanthanum the 4f orbitals become appreciably lower in energy than the 5d orbitals. Thus in the lanthanides, the electrons fill the 4f orbitals in a regular way.

iii) Similarly in Actinides also after Actinium, the 5f orbitals would become lower in energy than the 6d orbitals. Thus the electrons should enter the 5f-orbitals. However, for the first four elements Th, Pa, U, and Np the difference in energy between 5f and 6d orbitals is small.

Thus in these elements, electrons may occupy the 5f or the 6d levels or sometimes both. Later in the actinide series, the 5f orbitals do become appreciably lower in energy. Thus from Pu onwards the 5f shell fills in a regular way i.e. 6d1 electrons get shifted to 5f orbitals. The shifting is not seen in Cm and Lw because in Cm shifting gives unstable configuration [Rn]5f⁸ 6d⁰ 7s² while in Lw the shifting of 6d¹ electron to 5f orbitals is also not possible since 5f orbitals are already having 14 electrons.

Write notes on Oxidation states of Lanthanides

i) The most common and stable oxidation state of lanthanides is +3. The +3 state arises due to the loss of two electrons from the 6s orbital and one electron from the 5d or 4f orbital. The sum of the first three ionization energies is rather low.

ii) Apart from the +3 oxidation state, some lanthanides show +2 and +4 oxidation states. Such oxidation states are known as anomalous oxidation states. Such anomalous oxidation state is attributed due to the extra stability of empty, half-filled, and completely filled electronic configurations of f0, f7, or f¹⁴ electronic configuration.

iii) Some lanthanides show anomalous oxidation states of +2 and +4 even though they do not achieve stable, f0, f7, or f¹⁴ electronic configuration.

IV) Thus from the above discussion it is clear that although special stability is associated with empty, half-filled, and completely filled f-sub levels as an important factor, there are other factors like thermodynamic and Kinetic which are equally important for determining the stability of various oxidation states.

Explain the Magnetic properties of Lanthanides

i) Each electron in the substance act as a small micro-magnet. The magnetic properties of a substance represent the combined contribution of all the electrons present in the substance.

ii) An electron has both spin and orbital motion. Therefore the magnetic moment is due to the combined effect of spin and orbital moments of the electrons.

   iii) When the magnetic moment of tri positive ions is plotted against atomic number then a dotted curve is expected with a single peak at Gd³⁺ (7 unpaired electrons) should have obtained but instead a binodal curve is obtained with two peaks one at Pr³⁺, Nd³⁺ and other at Dy³⁺, Ho³⁺ region. This unusual curve is mainly due to the combined contribution of spin and orbital moments of electrons.

iv) From the binodal curve it is clear that, when the f-sublevel is less than half filled, the magnetic moments are less, because spin moments and orbital moments oppose each other. But when the f-sublevel is more than half-filled, magnetic moments are more, because spin and orbital moments do not oppose each other but act together.

Write a note on the ability to form complexes of lanthanides.

I) Lanthanide ions do not have a high charge-to-radius ratio, hence they do not have the tendency to form complexes. On the other hand transition, metal complexes have a high charge-to-radius ratio hence they have a high tendency to form a large number of complexes.

ii) The 4f electrons of lanthanides are much inside the atom and are well shielded by 5s and 5p electrons, hence 4f electrons hardly interact with the ligand field. Therefore ligand field stabilization energy is low and the complexes are less stable.

iii) In the complexes, the force of attraction between lanthanide ions and the ligands is mainly ionic.

iv) Lanthanide ions mainly form stable complexes with ligands having electronegative donor atoms like O and F, for example, EDTA, Citric acid, acetylacetone, and benzoyl acetone. On the other hand, it forms less stable complexes with the ligands possessing donor atoms like N, for example, ethylene diamine.

v) Known complexes of lanthanides,

  a) Ion pair associations: LnF₂¹⁺, LnSO₄^1+, and LnC₂O₄¹⁺

  b) Non Chelated species: LnI₃. xNH₃

  c) Chelated species: Chelated species are of two types:

              i) Non-ionic Chelated species : [Ln (A—A)₃] xH₂O

                                                              (A—A) = Acetyl acetone or benzoyl acetone

             ii) Ionic Chelated species : [Ln EDTA]^1-, [Ln (C₆H₅O₇)₂]^3-

vi) The stability constants of chelates are high and these increase with a decrease in the ionic radii of lanthanides. Between the adjacent Ln³⁺ ions, the stability constants differ considerably. This difference is helpful in the individual separation of lanthanides.

What is lanthanide contraction? Discuss the consequence of lanthanides contraction.

A steady decrease in the atomic and ionic radii of lanthanides with an increase in atomic number is known as lanthanide contraction.

Consequences or effects of lanthanide contraction

Decreasing Basicity of the lanthanides, [M(OH)₃]

Due to lanthanide contraction, the size of the +3 lanthanide ions decreases regularly with an increase in atomic number. As a result of a decrease in size, the covalent character between M3+ ion and OH- ion increases from La(OH)3 to Lu(OH)3. Therefore the basic character of the hydroxides decreases with an increase in atomic number. Thus La(OH)₃ is the most basic hydroxide while Lu(OH)₃ is relatively the least basic hydroxide.

These basicity differences are reflected in the hydrolysis of ions, the solubilities of their salts, the thermal decomposition of oxysalts, and the formation of complexes. It is due to differences in the above properties, the lanthanides can be separated by solvent extraction and fractional crystallization.

Variation in the properties of Lanthanides.

The properties of the lanthanides depend upon the size and charge. The characteristic oxidation state of all lanthanides is +3 and accordingly their chemical properties are very closely similar. However, due to lanthanide contraction in ionic radii, their marginal variation in size causes minor variations in properties. In fact, the chemical properties of these elements are so similar that their separation is very difficult.

Similarities between Yttrium and lanthanides.

In group III B, the atomic and ionic radii increase regularly in the order of  Sc < Y < La. However, from cerium onwards there is a slight but steady decrease in the ionic size, so much that the ionic size of Holmium (Ho³⁺ = 0.089 nm) and Erbium (Er³⁺ = 0.088 nm) becomes equal to that of Yttrium (Y³⁺ = 0.088 nm). Thus, yttrium ion has charges and sizes similar to those of heavier lanthanides.

Hence i) it occurs with heavier lanthanides, ii) its chemical properties become similar to heavier lanthanides, and iii) its separation from heavier lanthanides becomes difficult.

Similarities between Zirconium and Hafnium

In group IV B, the ionic radii are expected to increase in the order of Ti⁴⁺ < Zr⁴⁺ < Hf⁴⁺. But the ionic radius of Hf4+ (0.078 nm) is almost similar to that of Zr4+ (0.079nm). This similarity in ionic radii of Hf4+ and Zr4+ is due to the effect of lanthanide contraction on hafnium, which is the next element after lutecium. As zirconium and hafnium ions have similar charge and ionic radius, they have very much similar chemical properties and therefore are difficult to separate. Such a pair of ions is called a Chemical twin.

Similarly, there are other chemical twins like Nb-Ta, Mo-W, and Tc- Re having very similar chemical properties.

Abnormally high densities of post lanthanides

Due to the effect of lanthanides contraction, the atomic and ionic radii of post-lanthanides are very similar to those of the elements of the previous period. For example, Zr and Hf have similar sizes. But the atomic number and atomic weight of Hafnium are almost double that of Zirconium. Hence, the density of Hafnium is abnormally high. (Density of Zr = 6.59 g/cm3 and that of Hf = 13.3 g/cm3). Similarly, the other post-lanthanides, tantalum, tungsten, and rhenium have very high densities.

The noble character of post lanthanides

The post-lanthanides are found to be less reactive than their congeners in the second transition series. For example, Os, Ir, and Pt are less reactive than Ru, Rh, and Pd respectively. Similarly, Au and Hg are less reactive than Ag and Cd respectively.

Give any two important minerals of lanthanides.

Monazite, Cerite, Gadolinite and Xenotime.

Give the Application of lanthanides

In atomic Energy

Lanthanides like Gd, Sm, Eu, and Dy are used as moderators and neutron absorbers in atomic reactors to control the rate of fission. Long rods made up of these materials are introduced into the core of a reactor before the reactor is fueled. After the addition of fuel, if some rods are withdrawn, the fission reaction begins, and with the removal of more rods, the fission reaction accelerates.

If all the control rods are introduced in the core, the fission reaction stops. Isotopes of lanthanides have some potential use in atomic batteries, for the treatment of cancer, tracer’s studies, and sources of g -rays, and X-rays.

Commercial uses

Misch metal (Alloys of lanthanides) is a very good scavenger for oxygen and sulfur in several metallurgical operations. Misch metal has strong reducing properties. Magnesium alloys with misch metal and Zirconium possess high strength and resistance and are useful in jet engines. Misch metal also increases the resistance to nickel alloys and the working ability of stainless steel and vanadium.

Lanthanide oxides are dissolved in glass to impart beautiful colors to glass windows and glass vases

Mixed lanthanides are added to zeolites to enhance the catalytic activity, especially in the cracking of petroleum.

As Catalyst

Lanthanide oxides are used in the hydrogenation, dehydrogenation, and oxidation of various organic compounds. The chlorides of lanthanides and cerium phosphate are used in polyesterification processes and petroleum cracking. Anhydrous chlorides are used in polyesterification processes.

Magnetic and Electronic applications

Because of paramagnetic and ferromagnetic properties, lanthanides are useful in the field of the electronic industry. Ferromagnetic garnets, 3Ln₂O₃.5Fe₂O₃ are used in microwave devices and magnetic core materials.

Selenides and tellurides of lanthanides are used as semiconductors or thermoelectrics.

Europium-activated yttrium vanadate is used as red phosphors in colour television tubes. Some chelates of lanthanides are fluorescent or luminescent and may be useful in laser operation.

Informative links to read

Nanomaterials

Applications of Uranium

1. Metallic uranium and UC₂ can be used as catalysts in the synthesis of ammonia by

Haber’s process.

 2. Ferro-uranium alloy is used as the atomic clock which is a very accurate device.

3. UO₂ is used in powerful incandescent lamps for movie picture projection in photography.

4. Uranium salts are used as mordants for silk, wool, and Calico printing.

5. Uranium borate improves the appearance and increases the tensile strength of rubber.

6. Uranyl nitrate is used in medicine as anti-diabetes.

7. The most important application of uranium is in atomic energy. Uranium is a source of vast reservoirs of energy, which is harnessed for constructive purposes by the controlled chain reaction in nuclear reactors. On the other hand as a result of a controlled chain reaction, the energy released is used for a destructive purpose, which is the principle of the atomic bomb.

What is the Differences between Lanthanides and Actinides?

Informative links to read

Crystal field Theory | CFT | Crystal field splitting into octahedral and tetrahedral complexes.

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