Lanthanides or 4f – series preparation and properties, Group 18 d – and f – Block Elements
Lanthanides or 4f-series :
It includes 14-elementa i.e. from atomic number 58 to atomic number 71. In this series electrons enter in 4/-orbitals (perpenultimate orbital). These element are also known as rare earth elements because in past these elements were very rare but now a days this name is not appropriate as various elements of this series are not rare. Promethium (Pm) is the only element which is artificial and radioactive.
Electronic configuration of Lanthanides or 4f – series
→ Its general electronic configuration is (n – 2)f1-14 (n – 1)d0-1ns2 or 4f1-14ed0-16s2. As energies of 5d and 4/-orbitals are nearly similar hence their is various irregularities in their filling.
→ In the electronic configuration of all the elements of this occupancy. The electronic configuration of these elements having +3 oxidation state is similar i.e.4fn.
→ Where the value of n increases from 1 to 14 on increasing atomic number. The electronic configuration of ground state of Lanthanides is given in table 8.14
Table 8.14: Electronic configuration of Lanthanides with their stable oxidation state.
Some important striking features of electronic configuration of Lanthanides are as follow :
- 6s-subshell of these elements have two electrons but the number of electrons in 48-subshell is variable.
- The electronic configuration of Lanthanum (z = 57) is [Xe]5d16s2 i.e. Its f-subshell have no electrons. But it is considered that in the other elements off-block have electrons in their f-subshell.
Besides 5d1 in Gd and Lu, 4f-subshell is either half filled or completely filled hence the stability of these elements are higher
- Gd (z = 64) [Xe] 4f7 5d16s2
- Lu (Z = 74) : [Xe] 4f14 5d1 6s2
The stability of Eu (z = 63) and Yb (z = 70) is also higher due to half filled and full filled configuration.
- Eu (z = 63) : [Xe] 4f7 6s2
- Yb (z = 70) : [Xe] 4f14 6s2
Atomic and lonic radii (Lanthanide contraction)
In lanthanide series, there is a regular decrease in the atomic as well as ionic radii of trivalent ions (M ) as the atomic number increases from Cerium to Lutetium leaving some exceptions. This regular atomic and ionic radii on increasing atomic number is called lanthanide contraction. This decrease in size is very small. Atomic and ionic radil of Lanthanide series is given in table 8.15.
Table 8.15: Atomic and ionic radii of Lanthanides
|Element||Atomic raddi (pm)||Ionic radii (pm)|
Cause of Lanthanide contraction
→ In Lanthanide series, as atomic number increases, nuclear charge also increases by one unit and one electron also increases in 4/-orbitals of prepenultimate shell. But outer electronic configuration remain 5s2 5p6 6s2 ie. there is no change in outer electronic configuration.
→ On account of the very diffused shape of f-orbital, the 4f-electrons shield or screen each other quite poorly from the nuclear charge. Thus the effect of increased nuclear charge is somewhat larger than charged shielding effect.
→ Due to heigher nuclear charge than shielding effect, valane shell come close to nucleus, Hence the size of atom or ion goes on decreasing on increasing atomic number, However, this decraease in atomic radii is very small. This contraction is only about 10 pm from Ce to Lu.
Effects of Lanthanoid Contraction
→ Similarity in Atomic radius of one group in second (4d) and third (5d) transition series : The size of one group of 4d and 5d transition series is almost similar. In group 3 elements, there is regular increase in size from Sc to Y and from Y to La.
→ But after this, the size of one group in 4d to 5d series is almost same because Lanthanoids come in between them and size decreases due to Lanthanoid contraction. With increase in number of shells, atomic size stiould be increased but due to Lanthanoid contraction it is not so.
→ Separation of Lanthanoids: There is similarity in chemical properties of Lanthanoids due to Lanthanoid contraction, as a result, their separation is very difficult. There is difference in solubility, complex formation capacity of Lanthanoids due to Lanthanoid contraction.
→ Basicity of Lanthanoid hydroxides : With an increase in the atomic number, the basic strength of the oxides and hydroxides decreases. This contraction causes a decrease in the size of Lanthanoide cations and, therefore, the polrising power of the cations increases. This further decreases the ionic character of the oxides and hydroxides. Thus Ce(OH)4 is maximum and Lu(OH)4 is least basic.
→ Ionisation potential : The values of ionisation potential should be lower and should decrease regularly down the group. But due to Lanthanoide contraction there is a trend seen in the values of ionization potential, this regularity occurs after the element tungsten.
→ Oxidation States General oxidation state of all the Lanthanides is +3. However, occasionaly they show +2 and +4 oxidation state in solution or in solid compounds in the form of ions. This irregularities on oxidation states are due to extra stability of empty. half filled or fullfilled of f-subshel.
→ For example : Ce4+(4f0), Tb4+ (4f7), Eu2+ (4f7) Yb2+ (4f14) are very stable ions +2 or +4 oxidation states tend to revert to the more stable oxidation state of +3 Stable +3 oxidation state can be achieved by lossing or gaining an electron.
→ That is why Eu2+ and Yb2+ ions are very good reducing agents in solutions. On the other hand Ce4+ and Tb4+ ions are good oxidising agents in solutions. The E° value of Ce4+/Ce3+ and Tb4+ is +1.7 V which suggests that it can oxidise water. However, the reaction rate is very slow and hence Ce (iv) is a good analytical reagent.
→ The compounds of lanthanides are generally ionic in nature. There are some lanthanides which show +2 or +4 oxidation states yet they do not show f0, f7 and f14electronic configuration.
→ Examples : Pr4++ (4f1), Nd2+ (4f4), Nd4+ (4f2), Sm2+ (4f6), Dy4+ (4f8) etc. Pr, Nd, Tb and Dy also show +4 oxidation state but only in oxides, MO2. Europian forms Eu2+ ions by losing the two s-electrons and its f7 configuration accounts for the formation of this Eu2+ ion.
→ But it is a strong reducing agent and converts into Eu3+ form. Similarly Yb2+ (f14) is a reductant and Tb4+(f7) is an oxidant. Similarly like Europium exhibit both +2 and +3 oxidation state.
Genearl Characteristics :
The general characteristics of lanthanides are as follow:
Physical properties :
- All the lanthanides are silvery white metal but get tarnished rapidly in presence of air.
- They are good conductor of heat and electicity.
- Their density is generally high. the range of density is from 6.77 g cm to 9.74 g cm3. As atomic number increases density also increases.
- Their melting points are generally high. They melts at about 100 K to 1200 K temperature. The melting point of Smarium (Sm) is 1623 K.
→ Ionisation Enthalpy: Lanthanides have generally low ionisation enthalpies. The first ionisation enthalpies of the lanthanides are around 600 kJ mol-1. The second ioniation enthalpy is about 1200 kJ mol-1.
→ These values are quite comparable with those of calcium. Due to very low value of ioniation enthalpy, these elements are highly electropositive so these elements react cold and hot water to liberate hydrogen. However the reaction is slow with cold water but fast with hot water.
→ The standard reduction potential of these elements lie in between -22 to -24. Only Eu has the value -20 Due to favourable values of reduction potential these elements behave as strong reducing agent. Their reducing power decreases from La to Lu.
→ Colour: Many of the lanthanoid ions are coloured in solid as well as in solutions. The colour of Lanthanoid ions is due to f-f Transition as they have partially filled f-orbitals. The absorption bands of lanthanoid ions are narrow. It is because of the excitation within f-level. The lanthanoid ions, which do not have unpaired electron, are colourless. For example La3+ (4f0) and Lu3+ (4f14).
The colour fo some lanthanoid ions are given in table :
|Tb3+ (4/8)||Light pink|
→ Magnetic behaviour: The lanthanoid ions which have unpaired electrons are paramagnetic while the ions like La3+(f0), Ce4+(f0), Yb2+ (f14) and Lu3+ (f14) are dimagnatic due to the absence of unpaired electron.
→ Chemical reactivity : Lanthanoids are highly electropositive and these elements are quite reactive similar to calcium. But as atomic number increases they start to behave like aluminium. These metals get tarnished on exposure to air. On heating with air they form oxides of the type M2P3– Only cerium show exception as it from CeO2.
The chemical reactivity of lanthanoides can be represented as:
→ The values of first three ionisation energies of transition elements are very less. Hence, these elements are ionic and +3 is the most stable oxidation state in these compounds. Their chemistry is also depend on Ln3+ ion.
→ Reducing Property : Lanthanoides easily oxidise losing its three electrons and act as strong reducing agents.
Ln → Ln3+ + 3e–
→ Electropositive character : Their electron donating capacity is the indication of their strong electropositive character or metallic nature.
→ Reaction with water : These elements react with water and release hydrogen gas. With cold water, reaction is slow whereas it becomes fast with hot water.
2Ln + H2O → 2Ln(OH)2 + 3H2
The baisc nature of hydroxides decreases from Ce to Lu.
Reaction with Oxygen: These elements react with atmospheric oxygen and forms oxides.
2Ln + 3O2 → 2Ln2O3
Reaction with Hydrogen : These elements react with hydrogen at 300-400°C and forms non-stoichiometric hydrides like LnH2 and LnH3.
Reaction with Halogens:anthanides forms trihalides ion reacting with halogens.
2Ln + 3X2 → 2LnX3
With Non-metals : These elements react with non-metals like carbon, nitrogen and sulphur to form compounds.
Uses of Lanthanoids
Lanthanoides are used in various industrial processes. Some important uses are as follow :
- The alloys formed from Lanthanoides are known as Misch Metal.
- It contains 94-95% Lanthanides metals, 5% iorn and traces of sulphur, carbon, silicon, calcium and aluminium main lanthanide metal in misch metal is cerium which is about 40%. Lanthanum and neodymium also present in it.
- About the precentage of both is about 44%. These alloys are used for making tracer bullets, shells and flints for lighters.
- Cerium : Magnessium alloys are used in flash lights powers.
- An alloys of magnesium and 3% misch metal is used in making jet engine parts.
- 3% misch metal forms a strong alloy with magnesium and 1% zirconium, which has high strength and strong resistance at more than 3000°C. It is used in preparing jet engines.
In nuclear science : Because of high thermal properties, certain lanthanide elements are very useful in some nuclear applications. These are as follow:
- Gadolinium-titanium alloy is used in reactor shielding.
- European-samarium oxides have been dispersed in stainless steel control rods for use in nuclear reactor
- Thulium and samarium are used as portable X-ray sources.
In glass and ceramics:
- Cerium-neodymium oxides are used in goggles to filter out bright yellow sodium light in glass blowing processes.
- Lanthanum oxide is used in optical glass of high refractive index.
- Praseodymium-zirconium oxides are used in staining cermic tiles.
- Some oxides, carbides, sulphides of lanthanides are good high temperature resistant refractories.
As a catalyst :
- Cerium is used as catalyst in hydrocarbon oxygenation reactions.
- Lanthanium oxide is used for hydrogenation, dehydrogenation and oxidation of various organic compounds.
- Ceric sulphate and chloride are used as catalyst in petroleum cracking,
In magnetic and electronic instruments:
- Lanthanoids are useful in microwave devices.
- Lanthanoide selenide and tellurides are used in semi-conductors.
- Godolinium is used to produce low temperature by magnetic cooling during certain reactions.
- Cerium, praseudymium and samarium are used for preparing permanent magnets.
Miscellaneous uses :
- Lanthanoide fluoride is used in arc lamps as an alloy of carbon electrode that gives intense white light for searchlights and are lights.
- Neodymium-praseodymium oxides is used to produce atrificial gems.
- Europium oxide is used in picture tubes of colour televisions.
- Fluorescent lamps coated with phosphorus containing traces of europium and terbium oxides have higher light output and better colour balance.