Inner Transition Elements (f - block) - Lanthanoids

Lanthanides: Lanthanides consist of elements that follow lanthanum and involve the filling of 4f subshell.

Electronic Configuration: [Xe] 4fⁿ⁺¹ 5d° 6s² or [Xe] 4fⁿ 5d¹ 6s²

The general valence shell electronic configuration of lanthanides is 4f¹⁻¹⁴6s².

Atomic and Ionic Sizes of Lanthanides:

⇒ Atomic and ionic radii of lanthanides decrease with an increase in atomic number. This gradual decrease is known as lanthanides contraction.

⇒ Because of the lanthanides contraction, the radii of the elements of the 3^rd transition series are very similar to those of the corresponding elements of the 2^nd transition series elements.

Oxidation States of Lanthanides: Lanthanides exhibit the oxidation state of +3. Some of them also exhibit the oxidation state of +2 and +4.

⇒ a noble gas configuration e.g. Ce⁴⁺ (f⁰)

⇒ a half filled f shell e.g. Eu²⁺ (f⁷)

⇒ a completely filled f shell e.g. YB²²⁺ (f¹⁴) 

Lanthanide contraction:

⇒ It is observed that in lanthanide series, there is a progressive decrease in the atomic and ionic radii with increasing atomic number.

⇒ This regular decrease with increase in atomic number is called lanthanide contraction.

⇒ This is due to the weak shielding of f-orbitals.

⇒ These f orbitals are unable to counterbalance the effect of increasing nuclear charge because of which the size keeps on decreasing with increase in atomic umber.

Causes of Lanthanide Contraction:

⇒ As we move along the period from left to right in lanthanide series, the atomic number increases i.e. number of protons keeps on increasing.

⇒ For every proton added in the nucleus the extra electron goes to the same 4f orbital.

⇒ The 4f orbital shows poor shielding effect because of which there is a gradual increase in the effective nuclear charge experienced by the outer electrons. Thus, the attraction of the nucleus for the electrons in the outermost shell increases in atomic number.

Consequence of Lanthanide Contraction:

⇒ The elements that come after lanthanides are affected by lanthanide contraction. The atomic radii of period 6 elements is similar to that of elements of period 5. Thus, radii of period 6 elements are smaller than would’ve been expected in the absence of lanthanides.

⇒ Because of smaller size of period 6 elements, and the fact that they are very tightly packed in their metallic crystal that they have very high densities. The densities of elements in third transition series is almost double that of elements in the second transition series.

⇒ The ionic size of Y 3+ and Fr 3+ is similar with similar charges. There is a large similarity in the crystal structure, chemical properties and solubility between Yttrium compounds and compounds of other heavier lanthanides. Thus, Yttrium is regarded as a lanthanide.

Basic Strength of Hydroxide: Because of the lanthanide contraction, size of M³⁺ ions decreases and there is increase in covalent character in M–OH and hence basic character decreases. So, we see that La(OH)₃ is the most basic while Lu(OH)₃ is the least basic.

Similarity of 2nd and 3rd transition series i.e. 3d and 4d series: The atomic sizes of second row transition elements and third row transition elements are almost similar. This is also an effect of lanthanide contraction. As we move down the from form 4d to 5d series, the size must increase but it remains almost same due to the fact that the 4f electrons present in the 5d elements show poor shielding effect.

Complex formation: The lanthanides do not show much tendency to form complexes due to low charge density because of their size. However, the tendency to form complex and their stability increases with increasing atomic number.

Chemical Behaviour: The first few members of the series are quite reactive like calcium. However with increasing atomic number, their behaviour becomes similar to that of aluminum.

⇒ Lanthanides combine with hydrogen on gentle heating. When they are heated with carbon result in formation of carbides. On burning in the presence of halogens, lanthanides form halides.

⇒ Lanthanides react with dilute acids to liberate hydrogen gas.

⇒ Lanthanides form oxides and hydroxides of the type N₂O₃ and M(OH)₃ which are basic alkaline earth metal oxides and hydroxides.

Uses of Lanthanides:

⇒ Lanthanide are used in the production of alloy steels for plates and pipes.

⇒ Mixed oxides of lanthanides are used as catalysts in petroleum cracking industries.

⇒ Some lanthanum oxides are used as phosphors in television screens and other fluorescing surfaces.​