The f-Block Elements (Inner-Transition Elements)

The f-Block Elements (Inner-Transition Elements)

The periodic table places two rows of elements separately at the bottom: the Lanthanoids (Cerium, Z = 58, to Lutetium, Z = 71) and the Actinoids (Thorium, Z = 90, to Lawrencium, Z = 103). These are the f-block elements, also called inner-transition elements.

The defining feature of these elements is that the last electron added to each atom enters an f-orbital. Specifically, the electron fills the $(n-2)$f subshell. The general outer electronic configuration for these elements is:

This means that while the outermost $ns$ and $(n-1)d$ orbitals may have some electrons, the characteristic filling happens in the $(n-2)$f orbitals, which are two shells below the valence shell. This deep-seated filling is why their chemical properties within a series are so similar — the outer electron environment changes very little as you move across the series.

All f-block elements are metals. Within each series (lanthanoids or actinoids), the properties of the elements are remarkably similar. However, the chemistry of the early actinoids is more complicated than that of the corresponding lanthanoids. This complexity arises because actinoid elements can exhibit a large number of oxidation states.