Case: Read the passage and answer the following questions

The f-block consists of the two series, lanthanoids (the fourteen elements following lanthanum) and actinoids (the fourteen elements following actinium). . Because actinium closely resembles the actinoids, it is usually included in any discussion of the actinoids for which the general symbol An is often used. The chemistry of actinoids is complex. The complication arises partly owing to the occurrence of a wide range of oxidation states in these elements and partly because their radioactivity creates special problems in their study.

What are lanthanides and actinides?

Inner transition elements

The elements in which their elemental or ionic forms have partially filled f-orbitals are termed as f-block elements. These are known as inner transition elements because electrons in these elements enter into antipenultimate shell.

The elements in which 4f orbitals are progressively filled are called lanthanides.

The elements in which 5f orbitals are progressively filled are termed as actinides.