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In my book Chemistry Part I (NCERT XII), a statement on p. 232 goes like this:

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.

My thought on this matter is that $Z$ will increase and f-orbitals will shield electrons poorly and so ionisation enthalpy should increase. But I'm not able to understand what my book is trying to say. What is meant by less penetration to inner core of electrons? To be honest I wasn't able to understand all the above mentioned 3-4 lines. So, can anyone explain this properly to me, please?

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