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

1 Answer
Sep 3, 2016

Because both lanthanides and acitinides feature the filling of #f-"orbitals"#, which as valence electrons, give rise to some peculiar chemistry.


Just as the filling and partial filling of the #d-"orbitals"# gives rise to a distinct transition metal chemistry, the filling of the #f-"block"# gives valence orbitals that are extremely diffuse and multi-lobed. #f-"orbitals"# shield the nuclear charge VERY imperfectly. #f-"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.