Ozone
Ozone is a part of an on-going cycle in the atmosphere. It starts as UV radiation from the sun breaks the molecule bond of oxygen. The extra oxygen molecule, being unstable, wants to bond with O2, which is the more stable form. O3 is less stable than O2, so UV light quickly breaks the bond and the process starts again. When enough ozone is present in the atmosphere, it appears as a pale blue gas.
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