Step 1: Understanding the Concept:
Lanthanoids (\(4f\) series) and Actinoids (\(5f\) series) are the f-block elements located in the inner transition series.
Step 3: Detailed Explanation:
Similarities:
1. Both series show a predominant oxidation state of \(+3\).
2. Both series exhibit a steady decrease in ionic radii across the series, known as Lanthanoid contraction and Actinoid contraction respectively.
Differences:
1. Except for Promethium (\(Pm\)), lanthanoids are non-radioactive, whereas all actinoids are radioactive.
2. Lanthanoids have a lesser tendency to form complexes compared to actinoids because \(5f\) orbitals are more extended in space and participate more in bonding.
Step 4: Final Answer:
The primary similarities are OS and contraction; primary differences are radioactivity and complex formation.