<P> There are several known allotropes of oxygen . The most familiar is molecular oxygen (O), present at significant levels in Earth's atmosphere and also known as dioxygen or triplet oxygen . Another is the highly reactive ozone (O). Others include: </P> <Ul> <Li> Atomic oxygen (O, a free radical). </Li> <Li> Singlet oxygen (O *), either of two metastable states of molecular oxygen . </Li> <Li> Tetraoxygen (O), another metastable form . </Li> <Li> Solid oxygen, existing in six variously colored phases, of which one is O and another one metallic . </Li> </Ul> <Li> Atomic oxygen (O, a free radical). </Li>

List two forms of oxygen found in atmosphere
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