[bravoghost]

I just read in my chemistry book that the second electron in the 2s shell of alkaline earth metals mean that the Zeff is greater, this would mean that a second electron in the shell actually reduces the shielding? How is that possible? I thought that if a single electron in a subshell had a second electron added to its orbital, it would increase shielding and essentially weaken Zeff.

[opti384]

I just read in my chemistry book that the second electron in the 2s shell of alkaline earth metals mean that the Zeff is greater, this would mean that a second electron in the shell actually reduces the shielding? How is that possible? I thought that if a single electron in a subshell had a second electron added to its orbital, it would increase shielding and essentially weaken Zeff.

Do you mean that the Zeff is greater than the that of the alkaline metals?

[MrTeo]

Well, the electron added to the atom certainly gives a larger shielding to the positive charge of the nucleus, but there's also a positive charge added when you move from left to right in a period. Using Slater's rules we can easily see that with the increase of the atomic number the effective atomic number increases too (+0.35) but at the end of each period, due to the large shielding effect of the complete energetic level Z_eff decreases abruptly...

Anyway the explanation given by your book (maybe you should tell us the actual words the authors used) isn't clear at all, I understand your confusion