8.20 Compare the chemistry of actinoids with that of the lanthanoids with special reference to
(i) electronic configuration
(ii) atomic and ionic sizes
(iii) oxidation state
(iv) chemical reactivity.
(i) Electronic configuration
The general electronic configuration for lanthanoids and actinoids is ns2 (n-1)d0-1 (n-2)f1-14
Lanthanoids has 4f and actinoids has 5f, unlike 4f orbitals, the 5f orbitals are not deeply buried and participate in bonding to a greater extent.
(ii) Atomic and lonic size
Similar to lanthanoids, actinoids also exhibit actinoid contraction (overall decrease in
atomic and ionic radii). The contraction is greater due to the poor shielding effect of 5f orbitals.
(iii) Oxidation states
The principal oxidation state of lanthanoids is (+3). However, sometimes we also
encounter oxidation states of + 2 and + 4. This is 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 comparable energies. Again, (+3) is the principal oxidation
state for actinoids. Actinoids such as lanthanoids have more compounds in +3 state than
in +4 state.
iv. Chemical reactivity
In the lanthanide series, the earlier members of the series are more reactive. They have
reactivity that is comparable to Ca. With an increase in the atomic number, the lanthanides
start behaving similarly to Al. Actinoids, on the other hand, are highly reactive metals,
especially when they are finely divided. When they are added to boiling water, they give
a mixture of oxide and hydride. Actinoids combine with most of the non-metals at
moderate temperatures. Alkalies have no action on these actinoids. In the case of acids, they
are slightly affected by nitric acid (because of the formation of a protective oxide layer).