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Q & A: sodium ion size

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Most recent answer: 10/22/2007
Q:
Will you please explain why a na ion (+1) is smaller than a Na atom? -derek
- DEREK KO (age 14)
Seoul
A:

That's the results of the different orbital states that the electrons can go into. Quantum mechanics implies that the stable states an electron near a nucleus can be in don't have just any old size, but a set of special sizes- sort of like the different ways that a drum can vibrate ate fixed frequencies. Another implication is that at most two electrons (with opposite 'spins') can occupy one of those orbital states. The lowest energy ones fill up first. The states can be grouped into 'shells' which share, approximately, the same energy and size. A soudium atom has just one to many electrons to fill a shell, so the last one ends up in the next bigger shell. Stripping it off to make an Na+ ion leaves only the smaller shell.

Speaking of shells, I guess you might take all that as a short of shell game. I've taken your question and moved it under the 'quantum mechanics' shell. A more serious answer would try to develop the math behind the particular solutions, showing why the orbital shells exist and how many states are in each. That material is covered in a standard introductory quantum course.

Mike W.

(published on 10/22/2007)

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