In the periodic table, the term for the series of elements that fills the f-block is known as:

• A. Triads
• B. Representative elements
• C. Lanthanide series
• D. Bridge elements

Answer: (C)

The f-block contains the inner transition metals, where electrons fill the f-sublevels. Specifically, the elements that fill the 4f orbitals form the lanthanide series, which is the standard name for the group occupying the f-block. The actinide series also fills f-orbitals (the 5f subshell) but is a separate series lower on the table. The other terms don’t describe this particular group: triads refer to an old three-element grouping, representative elements denote the main-group s- and p-block elements, and “bridge elements” isn’t a standard label for a block. So the lanthanide series is the correct term.