Lanthanoids and Actinoids
The two rows of inner-transition (f-block) elements.
Lanthanoids
The 14 elements following lanthanum, characterised by the filling of 4f orbitals, and the lanthanoid contraction — a steady decrease in atomic/ionic size across the series.
Actinoids
The 14 elements following actinium, filling 5f orbitals — most are radioactive, and their chemistry is more complex than that of the lanthanoids.
Lesson 9 of 24