(a) In MnO, the oxidation state of Mn is +2 whereas in Mn2O7 it is +7. Lower oxidation states accounts for more no of unshared electrons in the form of lone pairs which can be donated and hence they act like lewis base. Whereas, the species in higher oxidation state has a lack of lone pair and they can accept the lone pairs and hence act like lewis acid.
(b) Lanthanoids have 4f orbital which has better shielding effect in comparison to 5f orbital of actinoids. Due to poor shielding of 5f orbital the effective nuclear charge increases and attracts the electrons of outermost shell with greater extent and reducing the size of element. So the actinoid contraction is greater than lanthanoid.
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