Classification of F- Block Elements

f- block elements consist of the 4f and 5f elements whose orbitals are constantly filled. The group 3 elements are the members of f block elements. 

They consist of two series: lanthanides and actinides. Lanthanides resemble each other more than any other transition elements in the series because they contain only one oxidation state. The excellent opportunity to obscure the effect of small changes in size and the nuclear charge is provided by their chemical properties and similar elements. 

Contrary to lanthanoids, actinides are much more complex elements, and their complication arises due to the presence of several oxidation states and their radioactive nature.

Lanthanides (4f series)

In the elements of this section, the last electron enters the 4f orbitals.

General characteristics

• Electronic configuration

 General configuration of lanthanoids

 [Xe] 4f¹⁻¹⁴ 5dº⁻¹ 6s²

 They contain three incomplete shells

• Oxidation states

• The oxidation state of all elements in the lanthanide series is +3. Some metals (Samarium, Europium, and Ytterbium) were previously thought to have +2 oxidation states. Further research on these metals and their derivatives has revealed that all metals in the lanthanide class have a +2 oxidation state in the solution.

• The use of two outermost electrons (6s²) and one inner electron. The inner electron used is a 5d electron, but the 4f electron can be used if not present.

• All lanthanides have +3 oxidation except cerium, praeseodymium, and terbium, which contain a +4 oxidation state.

• The +2 and +4 oxidation states only occur when:
  •   Noble gas configuration, e.g., Ce⁴⁺(f⁰)
  •   A half-filled f orbital, e.g., Eu²⁺, Tb⁴⁺(f⁷)
  •   Completely filled f orbital, e.g., Yb²⁺(f¹⁴)

Hence they act as an oxidising agent in a higher oxidation state and a reducing agent in a lower state. 

• Pr, Nd, Dy, and Tb exhibit a +4 oxidation state but only in oxides

Exceptions: some elements show +2 and +4 oxidation state without fº, f⁷, f¹⁴ configuration, e.g., Pr⁴⁺, Nd², Nd⁺⁴, Sm⁺² Dy⁺⁴, Tm⁺²⁺

• Magnetic properties 

In tri-positive ions, in the 4f subshell, the number of unpaired electrons keeps increasing from lanthanum to gadolinium. So lanthanum and lutetium are diamagnetic, and others are paramagnetic.

• Colour

Due to incomplete 4f orbitals, the lanthanide ions have unpaired electrons, and thus they absorb the energy of the visible region of light, undergo f-f transition, and show colour. The range of colour depends on the unpaired electrons in the 4f region.

• Other properties

• This group consists of highly dense metals with a high melting point.
• Ionisation energy – first, i.e., is 600 KJ/mole, the second is 1200 KJ/mole, third is low if it leads to a stable that is empty, half-filled, or full-filled orbital. This is the Reason why la, gd, lu has low third I.E.
• Electropositive character is high due to low i.p
• Due to the large size, they have low charge and thus do not have much tendency to form complexes.
• Good reducing agents
• Chemical behaviour

Earlier members of this series are pretty reactive; they behave more like Aluminium with the increasing atomic number.

Ln³⁺ (aq) + 3e⁻   →  Ln(s)

Ln burns in O2 become Ln2O3

Ln heated with S becomes Ln2S3

Uses

They are used in the production of alloy steels for plates and pipes. For example, mischmetal is an alloy of lanthanide metal used in mg-based alloy to produce bullets and shells.

Lanthanoid contraction

Due to increased nuclear charge and electrons entering the inner (n-2) f orbitals, the atomic size of ionic radii of tri positive lanthanide ions drops continuously from La to Lu. Lanthanide contraction is the progressive decrease in size as the atomic number increases.

Consequences of lanthanide contraction

1.  Lanthanides are challenging to separate due to minor differences in their atomic radii.
2. This also explains differences in properties such as the strong tendency for hydrolysis and the formation of complex salts, and the salts’ decreased thermal stability and solubility.
3. The second and third transition series elements have a similar size.

Applications of lanthanides

• Cerium is the most helpful element in the lanthanoid. 

CeO, La2O3, Nd2O3, and Pr2P3 is used as decolourising agent for glasses

• Lanthanoid compounds like cerium molybdate cerium tungstate are used as paints and dyes

Actinoids

Actinides are elements whose last electron enters the 5f orbital of the anti-penultimate shell. Natural actinoids- Th, Pa, and U, rest 11 are artificial.

General characteristics

• Electronic configuration

General configuration – [Rn] 5f¹⁻¹⁴ 6d⁰⁻¹ 7s²

• Oxidation states
  • Standard oxidation state – +3 as atomic number increases, oxidation state becomes more stable.
  • They show a more significant number of o.s than lanthanides.

• Other characteristics
  • silvery in appearance and have various structures
  • when finely divided, they are highly reactive
  • magnetic properties are more complex than lanthanides. Meaning, lower than early lanthanides because the 5f electrons are more effectively shielded from nuclear charge than 4f electrons of the corresponding lanthanides.

Conclusion

• Lanthanides are 4f series elements, while actinides are 5f series elements. 
• The general configuration of Lanthanides is [Xe] 4f¹⁻¹⁴ 5dº⁻¹ 6s². 
• On the other hand, actinides’ general configuration is [Rn] 5f¹⁻¹⁴ 6d⁰⁻¹ 7s². 
• Lanthanides’ most common oxidation state is +3, and actinides are +3. 
• Lanthanides have different colours due to unpaired electrons, while actinides are silvery in colour.