All the lanthanides have similar
outer electronic confugration and exibit common +3 Oxidation state in their
compounds, therefore lanthanides have exceedingly similar chemical properties.
Their similarity is much closer than that of ordinary transition elements
because lanthanides differ mainly in the number of 4f electrons which are
buried deep in the atoms of lanthanides and thus don’t influence their
properties. Moreover due to lanthanide contraction there is very small
difference in the size of these ions is almost identical which results in
similar chemical properties of these elements.