Re: Why do atoms 'want' a full outer shell?

Matt,

I’m not going to mince any words and go right to the point of your 
question.   The answer is because a full outer shell can provide the 
lowest level of electronic energy an atom can attain.  There is quantum 
theory to explain this (and which you will hopefully learn in the 
future), but the important thing is this has been experimentally 
verified. That means a full outer shell can give an atom its most stable 
electronic configuration.  Take a look at argon (Ar) on the periodic 
table: full outer shell, a “noble” gas, non-reactive except under extreme 
conditions.  It’s stable in its lower energy configuration.  It sees no 
reason to mess with it. (Matt, imagine if you will, you are on the couch 
with your Gameboy or Xbox; pizza and Coke on the coffee table. You are in 
your lowest energy configuration playing video games.  Why would you want 
to get up and start walking back and forth through the house bumping into 
things?) But look at sodium (Na) and chlorine (Cl).  Sodium has one less 
electron and chlorine has one more electron in their outer shells 
compared to argon. What is their situation?

An atom of sodium has one 3s electron outside a closed shell, and it was 
experimentally found to take 5.14 eV (“electron volts”) of energy to 
remove that electron – its “ionization potential”. The chlorine, on the 
other hand, lacks one electron to fill a shell, and releases 3.62 eV when 
it acquires that electron – its “electorn affinity”. It takes only 1.52 
eV of energy to donate the sodium 3s electron to chlorine when they are 
far apart, like infinity. When the atoms are brought closer together 
though, their electric potential energy becomes more and more negative, 
reaching -1.52 eV at about 0.94 nm separation. This means that if neutral 
sodium and chlorine atoms found themselves closer than 0.94 nm, it would 
be energetically favorable to transfer an electron from Na to Cl and form 
the “ionic bond”.

So, this electron transfer leads to: 
(1) The formation of a sodium cation (Na+) and a chlorine anion (Cl-).
(2) A dramatic and strong “electrostatic attraction” between them.
(3) Full outer shells with lower energy for both of them.

---* Dr. Ken Beck