Question

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what is the primary chemical responsible for the smell right before a
thunderstorm?
geosmin
ozone
methane
petricor

Explanation:

Before a thunderstorm, lightning can break apart oxygen molecules (\(O_2\)) in the air. Some of these oxygen atoms then combine with other oxygen molecules to form ozone (\(O_3\)). Ozone has a distinct smell that is often noticed right before a thunderstorm. Geosmin is associated with the smell of soil after rain (related to petrichor, which is a combination of geosmin and other compounds from plants and soil). Methane has a different, more like a "swamp - gas" smell and is not related to the pre - thunderstorm smell.

Answer:

Ozone