It is evident from the behaviour of the actinoids that the ionisation
enthalpies of the early actinoids, though not accurately known, but are
lower than for the early lanthanoids This is quite reasonable since it is to
be expected that when 5f orbitals are beginning 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 the 4f electrons
of the corresponding lanthanoids Because the outer electrons are less
firmly held, they are available for bonding in the actinoids