s, p, d, f subshells in multi-electron atoms

[Artin Allahverdian 2H]

What exactly is it that causes an s-electron to have lower energy than a p-electron in the same shell?

[Yousif Jafar 1G]

Due to the electrostatic interactions between the nucleus and the electrons in the various orbitals, the further the electron is from the nucleus, the higher its energy. Because the s-orbital is shielding the p-orbital in the same shell, the electrons of the p-orbital will have a higher energy because of the decreased effect of the nucleus.

[hazelyang2E]

This can also be explained through the concept of the effective nuclear charge which essentially says that because the inner electrons shield the outer electrons from electrostatic attraction of the positive nucleus, the outer electrons feel reduced electrostatic attraction. Because the s-electron is closer to the nucleus it shields the p-electron from the positive charge of the nucleus which then leads the s-electron having a lower energy level than the p-electron.

[bonnie_schmitz_1F]

In high school, I remember learning that the 4s- shell was before the 3d- shell because of how much larger the 3d- one was. Is this right?

[Artin Allahverdian 2H]

@ bonnie_schmitz_3K

Actually, 3d comes before 4s, given that 4s is fully occupied, because 3d has a lower energy state than 4s.

[Erin Nash - 4G]

3d always comes before 4s because it is at a lower energy level than 4s.

[Jordan Lo 2A]

I'm confused about this statement from the 6th Edition Ch 2 #37b homework problem:
Electrons in an s-orbital are more effective than those in other orbitals at shielding other electrons from the nuclear charge because an electron in an s-orbital can penetrate to the nucleus of the atom.
Is this because all the other orbitals other than s have nodal planes?