(i) Electronic configuration : Lanthanoids have general electronicconfiguration of [Xe]4f1−145d0−16s2and actinoids have general electronicconfiguration of [Rn]‌5‌f1−14‌6‌d0−1‌7‌s2. Thus, lanthanoids belong to 4 f-serieswhereas actinoids belong to 5 f-series.
Atomic and ionic sizes : The atomic size of lanthanoids decreasesfrom lanthanum to lutetium. Though the decrease is not regular, in caseof atomic radii, the decrease in the ionic size (M3+) is regular. Decrease insize between two successive elements is higher in actinoids due to poorscreening by 5f electrons.
(iii) Oxidation state : The most common oxidation state of lanthanoids is+3 while actinoids show more variable oxidation states than lanthanoidsranging from +3 to +7. The tendency of showing greater range of oxidationstates can be attributed to the fact that the 5f, 6d and 7s levels are ofcomparable energies and larger distance of 5f as compared to 4f from thenucleus.
(iv) Chemical reactivity : Actinoids are far more reactive than lanthanoids.They react with non-metals at moderate temperatures whereas lanthanoidsreact at high temperatures. Most actinoids are attacked by HCl but areslightly affected by HNO3 due to formation of a protective layer of oxideand alkalies give no reaction.
Lanthanoids liberate hydrogen from dilute acids and burn in halogens toform halides.