The chemical element oxygen is a non-metal with the atomic symbol O and atomic number 8, meaning every oxygen atom contains eight protons. While oxygen can exist in several elemental forms, the most common formula for elemental oxygen under normal conditions on Earth is O2. This diatomic molecule, often referred to as dioxygen, represents the stable, gaseous form that is essential for respiration in most life forms. It is the primary way the element is encountered in the atmosphere.
Understanding the Diatomic Molecule
The formula O2 signifies a diatomic molecule, consisting of two oxygen atoms chemically bonded together. This arrangement is highly stable because it allows both atoms to achieve a full valence shell, satisfying the octet rule. The atoms share two pairs of electrons, forming a strong covalent double bond.
The formation of this double bond releases a significant amount of energy, resulting in a molecule that is energetically favorable and much less reactive than its individual components. Molecular oxygen is a colorless, odorless, and tasteless gas at standard temperature and pressure. It constitutes approximately 21% of the Earth’s atmosphere and is continuously replenished through the process of photosynthesis. The stability of O2 is what permits its abundance and its function as the primary form of oxygen.
Distinguishing Atomic Oxygen
In contrast to the stable O2 molecule is atomic oxygen, represented by the formula O, which is a single, unbound oxygen atom. This form is extremely reactive and unstable because it has an incomplete valence shell, possessing two unpaired electrons. Atomic oxygen acts as a free radical and seeks to bond immediately to achieve stability.
Due to its short lifespan and high reactivity, atomic oxygen is not found at the Earth’s surface. It occurs naturally at very high altitudes, such as in the thermosphere and exosphere, where intense ultraviolet radiation causes the photo-dissociation of O2 molecules. While short-lived in most environments, it is the predominant species in low Earth orbit and is energetic enough to break chemical bonds on spacecraft materials.
The Triatomic Form of Oxygen (Ozone)
Oxygen also exists as ozone, which has the chemical formula O3. Ozone is an allotrope of oxygen, meaning it is a different structural form of the same element. Its molecule is triatomic, composed of three oxygen atoms bonded together in a bent shape.
The ozone molecule is less stable than O2 and readily decomposes back into molecular oxygen. This instability makes it a powerful oxidizing agent with a distinctive, pungent odor. Ozone is found primarily in the stratosphere, forming the ozone layer, which provides a protective shield by absorbing harmful ultraviolet radiation from the sun. When present in the lower atmosphere, or troposphere, this molecule is considered a toxic air pollutant.