Trend 8.1 of the Noble Gas (Group VIIIA) Trends says that all the Group VIIIA elements exist in the atomic form and are highly stable.

The stability of the noble gases comes from the fact that with the exception of He, the noble gases have filled s and p valence shells.

He: 〖1s〗^2 Ne: [He]〖2s〗^2 〖2p〗^6 Ar: [Ne]〖3s〗^2 〖3p〗^6 Kr: [Ar]〖4s〗^2 〖3d〗^10 〖4p〗^6 Xe: [Kr]〖5s〗^2 〖4d〗^10 〖5p〗^6 Rn: [Xe]〖6s〗^2 〖4f〗^14 〖5d〗^10 〖6s〗^6

Because the shells are completely filled, the electrons in the noble gases have the lowest possible, and therefore the most stable energy levels. And as a result, significantly more energy is required to ionize a noble gas, as can be seen when compared to the halogens which have high ionization energies themselves.

Noble gas kJ/mol Halogen kJ/mol
Ne: 2081 F: 1681
Ar: 1521 Cl: 1251
Kr: 1351 Br: 1140
Xe: 1170 I: 1010
Rn: 1037 At: 890

Another result of having filled valence shells is that with the exception of Xe under extreme conditions, the noble gases do not bond and do not form any compounds. The result of this is that the electronegativity of the noble gases has not been able to be measured and the noble gases occur naturally in their atomic form.

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