Inner transition elements

To learn about Inner transition elements

1. UNIT –V INNER TRANSITION ELEMENTS (Rare earths)

2. INNER TRANSITION ELEMENTS (Rare earths) The elements which have partly tilled (n-2) f- orbitals are df -block elements. Since their inner subshells (4f and 5f) are successively filled with electrons, these electrons are called transition elements. These are two series of inner transition elements. They are Lanthanide series (4f-block elements) Actinide series (5f-block elements)

3. LATHANIDES A group of 14 elements after lantanum (Z=58) consititutes anthanide series. Characteristic properties of lanthanides Electronic configuration: Lanthanides have 4f 2-14 5d 0-1 6s 2 ionic configuration . Oxidation states: The principal oxidation state of lanthanides is +3. In the case of La, Gd and Lu, removal of three electrons yield stable 4f , 4f 7 and 4f 14 configuration.

4. Lanthanide contraction On moving across the lanthanide series, the nuclear charge increases by +1 at each step and the addition of extra electron takes place in Colour and spectra: Some of the trivalent lanthanide ions having partly filled f- orbitals are coloured in the solid states well as in aqueous solution. The colour appears to depend upon the number of f-electrons. 4f orbital.

5. Magnetic properties The La 3+ (4f ), Ce 4+ (4f ), Yb 2+ (4f 14 ) and Lu 3+ (4f 14 ) ions have all the electrons paired and hence Oxidation potential: Lanthanides have high oxidation potentials. Thus, they have a strong tendency to lose their electrons to undergo oxidation. diamagnetic . They are strong reducing agents They show strong electropositive character

6. Basic character Lanthanides form trivalent ionic compounds. For example, their hydroxides. Ln (OH) 3 are ionic and basic in character. They are stronger base Complex formation: The lanthanide ions have low charge / size ratio, as compared to that of transition elements. They cannot, therefore, cause much polarization and consequently, they have poor tendency to form complexes. s than Al(OH) 3 .

7. Lanthanide contraction Definition : The atomic and ionic size usually decrease from left to right across a period. This is due to increase in effective nuclear charge (Z*) which pulls the orbital electrons closer to the nucleus . Cause of Lanthanide contraction: In lanthanide atoms and ions, the 4f orbital is filled successively from Ce to Lu. In general, the shielding effect of electrons decreases in the order ns > np > nf . Expression. Z * = Z – S

8. Consequence of lanthanide contraction Due to the close similarity in electronic configuration the lanthanides have identical chemical properties. The lanthanide contraction also explains the decreasing basicity of the lanthanides. Certain pairs of elements such as Zr / Hf , Nb /Ta and Mo/W have almost idential size against the expected size increase due to increased atomic numbers. This is a direct consequence of the lanthanide contraction.

9. Extraction of Lanthanides from Monazite sand Extraction of Lanthanide mixture Monazite ore is finely powdered and digested with hot conc.H 2 SO 4 in cast iron pans when thorium and lanthanides go into solution as their sulphate. The pasty mass of the sulphates is agitated with water and then filtered to remove silica. The formation of H 3 PO 4 makes the solution acidic. When it is neutralised with ammonia or magnesia mixture, thorium phosphate is precipitated and removed. The solution containing the sulphates of lanthanides is treated with Na 2 SO 4 when light lanthanides (La to Sm ) are precipitated as double sulphates while the heavier lanthanides ( Gd to Lu) remain in solution as simple sulphates .

10. The precipitated double sulphates ae treated with hot NaOH when hydroxides of light lanthanides are formed. The hydroxides ae washed and dried in air at 100 C to get oxides of light lanthanides. The oxides are treated with dil. HNO 3 when cerium is precipitated as CeO 2 . The solution contains nitrated of La, Pr, Nd and Sm. From this, the individual lanthanides are separated by a suitable method

11. Separation of Lanthanides Fractional crystallisation method The separation of lanthanide ions by this method is based on the slight differences in solubility. Fractional precipitation method The differences in stability of lanthanide complexes is the basic principle of this method Solvent extraction method This method works on the slight difference in the partition coefficients of the salts of lanthanides between water and an organic solvent. Ion exchange method This is the most elegant for lanthanide separation. It is based on the principle of selective exchange of lanthanide ions with the H + ion of a cation exchange resin.

12. Actinides The 14 elements from Th (Al.No.90) to Lw (At.No.103) are characterised by the progressive filling up of the 5f-orbitals. They const itute the actinide series. Characteristic Properties of Actinides Electronic configuration: The 5f block elements (actinides) have (n-2)f 1-14 (n-1) d 1 ns 2 configuration analogous to lanthanides .

13. Metallic nature: Like the lanthanides, actinides are all metals. Oxidation states: Unlike lanthanides, the actinides have no character oxidation state. Colour: Actinides cations having unpaired 5f electrons are coloured in the solid as well as in aqueous solution. Actinide contraction: Like the lanthanides, actinides show the phenomenon of actinide contraction. Magnetic properties: Like the lanthanides, actinides are strongly paramagnetic.

14. Transuranic elements Elements lying beyond uranium i.e. beyond atomic number 92 are called transuranic elements (Z = 93-103). These are radioactive elements produced through nuclear reactions. Neptunium: It was prepared by Wahi and Seaborg in 1942 by the action of high speed neutrons on U – 238. Plutonium: Plutonium – 239 is a very important isotope in nuclear chemistry. It is produced on a large scale (from natural uranium) by the action of slow neutrons on U – 238. Americium: It is prepared by bombarding Pu – 239 with alpha particles.

15. Comparison of Lanthanides and Actinides Points of Similarity Both show +3 oxidation state In both the cases, f- orbitals of (n-2) shell are progressively filled up A contraction in atomic and ionic size is observed in both the cases (lanthanide and actinide contractions) Both lanthanides and actinides show sharp line-like absorption spectra due to f-f transition They have low electronegativity values These elements are quite reactive These elements form coloured ions Their nitrates, perchlorates and sulphates in the +3 state are soluble in water. Their hydroxides and carbonates in +3 state are insoluble in water. Members of both the series show ion exchange behaviour

16. S.NO Lanthanides Actinides 1 They have identical chemical properties Variation is observed in their chemical properties 2 They have high binding energy Their binding energy is small 3 These elements exhibit oxidation states up to +4 These elements exhibit oxidation states up to +7 4 f-electrons have better shielding effect 5-electrons have poor shielding effect 5 The paramagnetic properties of the lanthanides can be easily explained. The magnetic properties of the actinides are quite complex 6 They hardly form complexes They form complexes readily 7 Except Pm, all are non-radio active All of them are radioactive 8 Their compounds are less basic Their compounds are more basic