The lanthanide elements are characterized by the filling of the 4f orbitals.

Here's why:

* Electronic Configuration: The lanthanides are filling their 4f orbitals. The general electronic configuration is [Xe] 4f¹⁻¹⁴ 5d⁰⁻¹ 6s².

* Similar Properties: The filling of the 4f orbitals leads to the lanthanides having very similar chemical properties. This is due to the 4f electrons being shielded from the nucleus by the outer 5d and 6s electrons.

* Contraction: The 4f orbitals are relatively small and have a poor shielding effect. This results in the lanthanide contraction, where the ionic radii of the lanthanides decrease gradually across the series.

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