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

[leright]

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

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?

Thanks.

 

[Gokul43201]

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).