Dear Lucas,
I don't know what your original question was about - I mean the specific gas element. In chemistry, everything is really determined by getting to those "stable states" you mentioned. Sometimes it occurs by elements "adding" electrons to their structure and sometimes by giving them up. I also don't know really how far into the "why" it happens you are prepared to go.
I think that an analogy may help. If you have ever seen or played on a "see-saw" or "teeter-totter" - it is a beam placed across a pivot point with seats on each end - it may be easier to understand. If only ONE person stands on one end, they must hold up the beam to keep it level. That takes energy, and the person will get tired soon. However, if another person gets on the other end, they can sit so that their weights balance the beam and both can sit comfortably without using energy. The actual toy is used for fun by bouncing it up and down like a ride, but that also can ONLY be done when there are two people. One person cannot "play" on a see-saw alone.
The single person is like that atom. It exists, but it would prefer to exist in a more "stable" state that requires less energy to maintain. "Adding" the additional person might even take some initial energy effort (activation energy in chemistry), but the result is a MORE stable arrangement, requiring LESS energy to maintain than the original condition with just one person (or electron).
If you are curious about the actual energetic relationships in atomic structure and chemical bonding you can continue to read about it in chemistry textbooks at various academic levels or take chemistry courses from some schools. It is very interesting to many of us, but it is not something that you can just browse and understand with a few quick examples.
Happy learning!
Steven
