# Lanthanoid and actinoids are placed in separate rows at the bottom of the periodic table. What is the reason for this arrangement?

Because both lanthanides and acitinides feature the filling of $f - \text{orbitals}$, which as valence electrons, give rise to some peculiar chemistry.
Just as the filling and partial filling of the $d - \text{orbitals}$ gives rise to a distinct transition metal chemistry, the filling of the $f - \text{block}$ gives valence orbitals that are extremely diffuse and multi-lobed. $f - \text{orbitals}$ shield the nuclear charge VERY imperfectly. $f - \text{block}$ metals are thus highly Lewis-acidic, and they tend to form ${M}^{3 +}$ ions (though other oxidation states are known). The similar chemistry across the 2 periods is sufficient to warrant grouping these lanthanides and actinides together.