Valence Electrons: Why Move from Group 1 to 17?

[Infrasound]

What causes an electron to move from a group 1 element to a group 17?

I understand that opposite charges attract, but why would the valence electron of sodium be more attracted to the nucleus of chlorine than its own?

It seems strange that this occurs if the radius of a chlorine element is so much greater than that of sodium. Doesn't electrical force decrease at a rate equal to the inverse of square of the distance from the nucleus?


Also, do we understand why electrons exist in specific energy levels? As opposed to anywhere like planetary orbits. Is this one of those quantum physics realms where I would have to understand the mathematics?

Thanks in advance for any help with either question.

 

[kanato]

Yeah, these are mostly due to quantum effects.

Electrons in an atom can, to a good approximation, be divided into core electrons and valence electrons. The core electrons are very tightly bound to the nucleus, and located very close to it, whereas the valence electrons are located further out. So the core electrons end up doing an effective job at screening the nuclear charge, so roughly speaking, as far as the valence electron is concerned, it will see a charge of Z - N_{\mathrm{core}}. In sodium, it has a single valence electron and 10 core electrons, so the effective charge that the valence electron sees is just 1. A Cl atom has the same 10 core electrons, but the nuclear charge is 17, so the effective charge the valence electrons see is 7. This actually results in the Cl atom being smaller than the Na atom.

Now, you ask why do some of the electrons behave like core electrons and others behave like valence electrons, the answer is due to quantum mechanics. QM results in discrete energy levels, the highest of which are the valence which are significantly higher in energy than the core energy levels. And once you count up states, you end up with the "noble gas rule," where noble gas configurations end up being the most stable. These occur at 2,10,18,36,... total electrons, so for Na and Cl it turns out Na is more stable at Na+ with 10 electrons, and Cl is more stable at Cl- with 18 electrons. (That is, a system containing an Na+ with a Cl- is more stable than a system with a neutral Na with a neutral Cl.)