The 4s orbital, however, is more penetrating than the 3d orbital. (The 4s orbital has 3 radial nodes and a 3d orbital has none). An electron in a 4s orbital thus has a fairly high probability of being closer to the nucleus than the core electrons. The core electrons shield the nuclear charge from a 4s electron less than they do for a 3d electron. A 4s electron experiences a higher Zeff than a 3d electron and so it is better to occupy the 4s orbital in K. (When there are no core electrons to penetrate - as in the hydrogen atom - the penetrating power of an orbital is irrelevant).
(from the same website)
The 4s orbital has 3 radial nodes and a 3d orbital has none
What is the relationship between the nodes and penetrating power?
(I think I have mixed-up)
4s is more penetrating than 3d orbitals, what's that mean?
It means the e- in 4s orbital has more chances to penetrate the core electrons to the nucleus?