Why is chemistry of all lanthanoids identical ?

Explanation

All the lanthanoids have similar outer electronic configuration and show +3 oxidation state in their compounds. Therefore, all the lanthanoids have similar chemical properties. The different lanthanoids differ mainly in the number of 4f-electrons which are burried deep in the atoms and hence, do not influence the properties. Moreover, due to lanthanoid contraction, there is very small difference in the size of all the trivalent lanthanoid ions. Thus, the size of their ions is also almost identical which results in similar chemical properties.