When two separate oxygen atoms combine with two chemical bonds, the result is one molecule of oxygen. A stable compound forms when two oxygen molecules share two pairs of electrons with each other.
Each atom has eight protons, eight electrons and eight neutrons. The inner ring has two electrons, while the outer ring has six. The maximum an electron ring can hold is eight, though the first ring can hold only two.
As the atoms are bound together in a molecule of oxygen, the outer level of energy shares two of its electrons with a neighboring atom. This gives both atoms an outer layer of eight electrons.
Gases have an innate reaction to become noble gases, which is achieved by acquiring the eight electrons. Due to oxygen's electronegativity, or ability to magnetically attract electrons from neighboring atoms, it combines with almost all other elements in an attempt to complete the outer energy level. Metals, such as iron, aluminum or titanium, rust when they come in contact with oxygen atoms. This creates an outer layer of oxygenated compounds that eventually decays the metal.
Oxygen is the most abundant element on earth and, after hydrogen and helium, the third most common element in the Milky Way galaxy.