Oxygen occurs as two allotropes, dioxygen and trioxygen (ozone). Dioxygen, O2, is a diatomic colourless oxidizing gas but a pale blue liquid upon condensation. It is paramagnetic (that is, possesses unpaired electrons) as a result of two unpaired electrons in the two O2 π* orbitals. It makes up about a fifth of the earth's atmosphere.
Ozone, O3, is a very reactive blue gas characterized by a pungent "electrical" smell (the name ozone derives from the Greek ozein, to smell). Ozone molecules are bent with a bond angle of about 117° in the gas phase. Ozone condenses to a blue-black liquid (boiling point -111.9°C and further cooling results in a dark violet-black solid (melting point -192.5°C.
Ozone is unstable thermodynamcally with respect to dioxygen but only converts to O2 slowly in the absence of a catalyst.
2O3(g) → 3O2(g)
Ozone is present in the atmosphere as well, but at amuch lower level than O2. Ozone is a key component of the upper atmosphere because it has the ability to absorb ultraviolet light in the 220-290 nm range, so preventing harmful rays at those wavelengths reaching the earth's surface. These days, various factors deplete the ozone layer in the upper atmosphere and the "ozone holes" in the northern and southern polar regions are of particular concern.