f-block Elements | Actinoids and their Properties#

Actinoids#

The actinoids include fourteen elements from Thorium (Th) to Lawrencium (Lr).

General Electronic Configuration

\[(n-2)f^{0-14} (n-1)d^{0-1}ns^2\]

The irregularities in the electronic configuration of actinoids are related to stability of f⁰, f⁷ and f¹⁴ configuration of the 5f orbitals.
The chemistry of actinoids is much complicated due to: (a) wide range of oxidation states (b) Radioactive property

Atomic and Ionic Sizes#

Actinoid Contraction

The gradual decrease in the size of atoms or M³⁺ ions across the actinoid series is called actinoid contraction.

Actinoid contraction is greater from element to element than lanthanoid contraction due to very poor shielding of 5f-electrons as compared to 4f-electrons.

Oxidation States#

In general, actinoids show +3 oxidation state.
However, they exhibit a very wide range of oxidation states, which makes their chemistry complicated.

Similarities between Lanthanoids and Actinoids#

In both, f-orbitals are progressively filled.
Both form coloured ions.
Both are paramagnetic in nature.
Both show contraction, ie, lanthanoid contraction and actinoid contraction.

Differences between Lanthanoids and Actinoids#

S.No.	Lanthanoids	Actinoids
1.	4-f orbital is progressively filled.	5-f orbital is progressively filled.
2.	+3 oxidation state is the most common along with +2 and +4.	They show +2, +3, +4, +5, +6, +7 oxidation states.
3.	Only promethium (Pm) is radioactive.	All are radioactive.
4.	They are less reactive than actinoid.	They are more reactive.
5.	The magnetic properties are less complex.	The magnetic properties are more complex.

Applications of d and f block Elements#

Tungsten (W) is used in making electrical filaments.
Titanium is used in nuclear reactions.
Iron is used as a catalyst in Haber's process.

Question#

Actinoid contraction is greater from element to element than lanthanoid contraction. Why?

It is due to very poor shielding of 5f electrons as compared to 4f electrons.