In my textbook it is explained that 4s electrons show greater penetration than that of 3d electrons, and, thus, the energy of the 4s orbital is lower than that of the orbitals in a 3d energy sublevel. Here is what my textbook (General Chemistry Principles and Modern Applications, Tenth Edition) says exactly:

"For example, because of the extra penetration of a 4s electron over that of a 3d electron, the 4s energy level is below the 3d level despite its higher principal quantum number n..."

I took this to mean that 4s electrons are closer to the nucleus than that of 3d electrons. Is this true?

Furthermore, after doing research on why electrons are removed from the 4s orbital before the 3d orbital (a topic that still confuses me) I ran into a source that seemingly contradicts what my textbook says:

"When you talk about ionisation energies for these elements, you talk in terms of the 4s electrons as the outer electrons being shielded from the nucleus by the inner 3d levels."

From: http://www.chemguide.co.uk/atoms/properties/3d4sproblem.html

This sounds like it contradicts my textbook. Perhaps it does not. However, it does raise a few questions (one of which I posed earlier). Does the term "penetration" mean that the 4s electrons are closer to the nucleus than the 3d electrons? If not, what does it really mean? Also, what distinction is there between the aufbau process and ionization regarding differences in energies between 4s and 3d orbitals?


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