9.0 Actinoids Series

  d and f Block Elements

• The elements in which last i.e.,differentiating elements is entering in pre-penultimate $5f$ subshell are called actinoids.

• The group of $14$ elements from the $Th$ ($Z=90$) to $Lr$ ($Z=103$) in which last electron is entering in $5f$ subshell are called actinides or actinoids or actinones.

• They are called actinoids because their properties are similar to theactinium which a prototype of this series.

• These elements constitute second transition series.

• They have identical electronic configuration in valence and penultimate shell, hence their properties are closely similar.

• Only thorium and uranium are naturally occurring whereas remaining are prepared in laboratory.

• The elements having atomic number higher than $92$ and which are synthetically prepared in laboratory by nuclear reactions are called trans uranic elements.