Answer:
Ce, Pr and Nd are
lanthanoids and have incomplete 4f shell while Th, Pa and U are actinoids and
have 5f shell incomplete.
In the beginning,
when 5f - orbitals begin to be occupied, they will penetrate less into the inner
core of electrons. The 5f-electrons will therefore, be more effectively shielded
from the nuclear charge than 4f electrons of the corresponding lanthanoids.
Therefore, outer
electrons are less firmly held and they are available for bonding in the actinoids.
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