Elemente si combinatii chimice

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Allotropic states. Oxygen has two allotropic states: O2 – di-oxygen, and O3 – tri-oxygenor ozone.
Reaction of forming O3 from O2 is endothermic and its
reverse is exothermic:
3O2 → 2O3 ΔH = +8.1 kJ·mol-1 2O3 → 3O2 ΔH = -142 kJ·mol-1 O3 results also from thermal dissociation of O2 at over
1500°C, when O2 dissociates in 2 atoms of O with which
O2 leads to O3. The action of the ultraviolet radiation (UV) on O2 produces traces of O3 in the upper atmosphere
(stratosphere). Highest concentration of ozone are about
25Km altitude. The presence of the ozone is of vital
importance for protecting Earth’s surface on excessive
exposure to UV radiations.