In the atmosphere

Oxygen has electronic configuration 2, 6. The valence shell of oxygen has 6 electrons so its valency is 2. It wants to accept two electrons to complete its octet to get stability. So, it combines with another oxygen atom to complete its octet. In the O₂ molecule, both O atoms have a complete octet. So O₂ is more stable than an O atom. Hence Oxygen is available as O₂ in the atmosphere.

OR

Two uses of isotopes are-

i. Radioactive isotopes are used as tracers in medicine to detect the presence of tumors, blood clots in the human body.

ii. They are used in industry to detect leakage in underground oil, gas, or water pipelines.