ozone — a compound whose molecules contain three oxygen atoms. Ozone
(O3) differs from oxygen gas
(O2), whose molecules contain two oxygen atoms. Ozone is a gas,
and in the upper atmosphere it protects the earth from the sun's ultraviolet radiation and can
be depleted by chlorofluorocarbons. Closer to the ground it can be a pollutant. Ozone is also
used in water treatment.