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Why do atoms prefer filled, half filled, or empty shells?

by leright
Tags: atoms, filled, prefer, shells
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Oct30-06, 02:28 PM
P: 1,194
What is the reason for this?? In chemistry, they tell you that atoms are "happier" in these states, but WHY!?! Nobody can seem to give me an answer. I have a hunch that it has to do with entropy of the various configurations, but can someone explain this to me?

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Oct30-06, 03:28 PM
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I'm guessing you mean "subshells".

The reason for the relative stability of a half-filled subshell is the fact that beyond half-filling, you start to put in a second electron into an orbital, increasing its Coulomb energy (alternatively, you can think of this as an increase in the "spin-spin interaction" energy needed to maintain a pair of opposite spins in the same orbital). As for the loss of stability often seen in starting to fill a new subshell, this can be thought of in terms of screening. Electrons within the same subshell are not as good at screening nuclear charge as electrons in a lower subshell. So, when you go from Be to B (for instance), the most energetic electron actually sees a decrease in effective nuclear charge, making it less tightly bound. Hence the lower ionization energy for B compared to Be (opposing the periodic trend).

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