(i) Oxidation states :

The major oxidation state of lanthanoids is (+3). However, sometimes we also find oxidation states of +2 and +4. This occurs because of the extra stability of fully-filled and half-filled orbitals. Actinoids exhibit a greater range of oxidation states. This is because the 5f, 6d, and 7s levels are of equivalent energies. Again, (+3) is the principal oxidation state for actinoids.

(ii) Chemical reactivity: In the lanthanide series, members appearing earlier in the series are more reactive. With an increase in the atomic number, the lanthanides start behaving similar to aluminium. Actinoids, on the other hand, are highly reactive metals, especially when they are finely divided and they provide a large surface area for chemical reactivity. When they are added to boiled water, they yield a mixture of oxide and hydride. Actinoids combine with most of the non-metals at moderate temperatures. Alkalis have no action on these actinoids. In case of acids, they are slightly affected by nitric acid (because of the formation of a protective oxide layer).