[Sophia7X]

For the element with the electron configuration
1s22s22p63s2, one of the 3s electrons will be shielded
from the nuclear charge most effectively by a...

(A) 1s electron (B) 2s electron
(C) 2p electron (D) 3s electron

The answer is supposed to be A. I guessed C (not really sure why), could someone explain? Thanks.    Btw, this was one of the problems I got wrong on the national chem olympiad exam (didn't make it into the study camp, but at least I got honorable mention). Oh well, there's next year to try again.

[ramboacid]

Btw, this was one of the problems I got wrong on the national chem olympiad exam (didn't make it into the study camp, but at least I got honorable mention). Oh well, there's next year to try again.

Haha yep, this was one of them I missed too. And yep there's always next year

The reason is simply that electrons closer to the nucleus are the most penetrating and thus block out the nuclear charge the most. There is a pdf I read online that explicitly states this, though I can't find it now. ChemWiki does a good job of explaining it though:
http://chemwiki.ucdavis.edu/index.php?title=Physical_Chemistry/Quantum_Mechanics/Quantum_Theory/Trapped_Particles/Atoms/Multi-Electron_Atoms/Penetration_%26_Shielding
The pertinent part is reproduced below.

Shielding
Shielding describes the amount of screening from nuclear charge that one electron can do with respect to its neighboring electrons. Electrons that have greater penetration can get closer to the nucleus and effectively block out the charge from electrons that have less proximity.  For example, Zeff is calculated by subtracting the magnitude of shielding from the total nuclear charge. The value of Zeff will provide information on how much of a charge an electron actually experiences.

Because the order of electron penetration from greatest to least is s, p, d, f; the order of the amount of shielding done is also in the order s, p, d, f.

[Sophia7X]

Thanks!

[cheese (MSW)]

Research Slater's Rules and calculate the Zeff for a 3s e⁻ in Mg (Ans = 2.85).
Then rethink your answer.