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Ionisation enthalpies of early actinoids are lower than for early lanthanoids.
Reason: Electrons are entering 5f orbitals in actinoids which experience greater shielding from nuclear charge.
A
Both A and R are true and R is the correct explanation of A
B
Both A and R are true but R is NOT the correct explanation of A
C
A is true but R is false
D
A is false but R is true
Detailed Solution
The ionisation enthalpies of early actinoids are indeed lower than those of the corresponding early lanthanoids, because electrons entering the 5f orbitals in actinoids experience greater shielding from the nuclear charge than the 4f electrons in lanthanoids do, which correctly explains the lower ionisation enthalpy.