[chromium184]

Can someone possibly explain this to me? I understand the general trends with penetration and shielding, but I forget why the the 3s orbital electrons have a lower Zeff than the 3p, versus the 2s and 2p. I should definitely know this.. I just forget.

[pnicogen1]

Anyone correct me if I'm wrong, but the reason why Zeff increases from left to right is because the atomic number (number of protons) increases from left to right. You add one more proton to the nucleus as you move right and you add one more electron. The reason why overall the Zeff increases (despite adding another e-) is because the e- don't shield eachother perfectly, so overall you have a stronger Zeff. You will see this trend also if you are working with Slater's rules, that the e- don't shield perfectly depending on where they are in the orbitals.

[lokifenrir96]

Actually, I thought Zeff (effective nuclear charge) is just the actual nuclear charge (no. of protons) - no. of shielding electrons (no. of inner electrons, i.e. total no. of electrons - valence electrons). So the reason why it increases from left to right, across the period, is because the no. of protons increases, but the electrons are added to the valence shell, so the no. of inner electrons remains the same. This is just what I was taught, I'm not sure if your explanation is also valid, pnicogen.

What I don't understand, though, is why electrons in p orbital have weaker shielding effect than those in s, and d weaker than those in p, and so on. Is it perhaps because of the shape of the orbital and how diffuse it is?