What is the standard oxygen we breathe classified as?

The standard oxygen that we breathe is classified as O2, which refers to molecular oxygen composed of two oxygen atoms. This form of oxygen is essential for the respiration of most living organisms on Earth. When we breathe in air, approximately 21% of that air is O2, which our bodies utilize in cellular respiration to produce energy.

In contrast, O3 is ozone, a molecule consisting of three oxygen atoms, which is found in the Earth's stratosphere and plays a key role in blocking harmful ultraviolet radiation but is not suitable for respiration. CO2 refers to carbon dioxide, a waste product of respiration that is exhaled. N2 is nitrogen, which makes up about 78% of the Earth's atmosphere; however, it is not used by the human body for respiration. Therefore, O2 is the only choice among the options that accurately represents the oxygen we need to survive.