Summary:
- Helium has a full outer shell of two electrons.
- The other gases have full outer shells of two and six electrons.
- Compounds have been formed with Rn, Xe, Kr, and Ar.
- The first compound (XePtF6) was made with xenon in 1962.
- Radon also will give a reddish glow, but is not used because it is radioactive and will not retain its structure as radon for any significant length of time.
- Only helium and neon have not formed compounds at this time.