Which block elements are known as inner transition elements?

f-block

The correct answer is option 4. f-block.

Inner transition elements are a subset of transition metals found in the f-block of the periodic table. The f-block is characterized by its position below the main body of the periodic table, separated from the rest of the elements. The f-block consists of two series of inner transition elements:

1. Lanthanides: The first series of inner transition elements is the lanthanides, which include elements with atomic numbers 57 to 71. The lanthanides are often referred to as the "rare earth elements." Examples include cerium (Ce), europium (Eu), and erbium (Er).

2. Actinides: The second series is the actinides, which include elements with atomic numbers 89 to 103. The actinides are known for their radioactive nature. Prominent members of this series include uranium (U), thorium (Th), and plutonium (Pu).

The f-block elements have partially filled f orbitals, which are part of the inner electron shell. These inner electrons are shielded by outer electron shells, making the chemistry of the f-block elements unique. The f-block elements are further categorized as inner transition elements due to their electron configurations and distinctive properties.

In summary, the f-block elements, including lanthanides and actinides, are known as inner transition elements.