In the universe, oxygen is the third most abundant element after hydrogen and helium. Oxygen is synthesized at the end of the life of massive stars, when helium is fused into carbon, nitrogen, and oxygen nuclei. Stars burn out, explode, and expel the heavier elements into interstellar space. Later, oxygen plays a crucial role in the emergence of life. Oxygen is not always reactive to the same extent. The oxygen atom (O) is more reactive than the oxygen molecule (O2). To understand this, we will review some of the basics of oxygen. The oxygen atom is depicted in Fig. 9.1.