Allotropes of oxygen

There are several known allotropes of oxygen. The most familiar is molecular oxygen (O₂), present at significant levels in Earth's atmosphere and also known as dioxygen or triplet oxygen. Another is the highly reactive ozone (O₃). Others are:

• Atomic oxygen (O₁), a free radical.
• Singlet oxygen (O^*
  ₂), one of two metastable states of molecular oxygen.
• Tetraoxygen (O₄), another metastable form.
• Solid oxygen, existing in six variously colored phases, of which one is octaoxygen (O₈, red oxygen) and another one metallic (ζ-oxygen).