Oxygen, the eighth element on the periodic table, is highly reactive and forms chemical bonds with nearly every other element. This reactivity allows oxygen to participate in countless compounds, shaping the environment and all known forms of life. Its abundance and ability to readily combine with other substances make it a central player in the chemistry of our planet.
How Oxygen Forms Compounds
Oxygen’s ability to form a vast array of compounds stems from its unique electron configuration and high electronegativity. Oxygen atoms have six electrons in their outermost shell and aim to achieve a stable configuration of eight electrons. This often leads them to form two chemical bonds, either by sharing electrons in covalent bonds or by gaining electrons to form an ion in ionic bonds.
Because of its strong attraction for electrons, oxygen is the second most electronegative element, surpassed only by fluorine. This strong electronegativity means oxygen pulls electrons closer to itself when forming bonds, creating a partial negative charge on the oxygen atom. This property drives its reactions with other elements, leading to diverse compounds.
Everyday Inorganic Oxygen Compounds
Oxygen combines with other elements to create numerous inorganic compounds, many of which are familiar in daily life. Water (H₂O), composed of two hydrogen atoms bonded to one oxygen atom, is perhaps the most ubiquitous example. Water covers about 71% of Earth’s surface and is essential for all known life forms, acting as a universal solvent for many substances. Its unique properties, such as high surface tension and ability to dissolve many compounds, are due to oxygen’s electronegativity creating a polar molecule.
Carbon dioxide (CO₂), a molecule with one carbon atom double-bonded to two oxygen atoms, is another common inorganic compound. This gas is a trace component of Earth’s atmosphere, playing a role in the greenhouse effect and the carbon cycle. Plants utilize carbon dioxide during photosynthesis to produce sugars and release oxygen, while living organisms release it during respiration.
Beyond water and carbon dioxide, oxygen forms a broad class of compounds known as oxides, which are combinations of oxygen with another element. Iron oxide, commonly called rust, forms when iron reacts with oxygen and moisture. Silicon dioxide, or silica, is another prevalent oxide found in sand and quartz.
Oxides can be metallic, like aluminum oxide used in ceramics, or non-metallic, such as sulfur dioxide, which contributes to volcanic gases and air pollution. Ozone (O₃) is an allotrope of oxygen, meaning it is a different structural form of the element. This molecule consists of three oxygen atoms bonded together, making it less stable than the more common diatomic oxygen (O₂). High in the stratosphere, ozone forms a protective layer that absorbs most of the sun’s harmful ultraviolet (UV-B) radiation, shielding life on Earth.
Oxygen in Living Things and Organic Molecules
Oxygen is a component of the molecules that make up living organisms, playing a role in organic chemistry and biological systems. Many major classes of biological macromolecules, including carbohydrates, lipids, and proteins, contain oxygen atoms. These molecules are essential for the structure, energy flow, and function of cells.
In carbohydrates, oxygen is present in the hydroxyl (-OH) groups, which are characteristic of sugars and starches. Lipids, such as fats and oils, also contain oxygen in their ester linkages. Proteins incorporate oxygen in their peptide bonds and in the carboxyl (-COOH) and hydroxyl groups of amino acid side chains. These oxygen-containing functional groups contribute to the diverse shapes and functions of these biological molecules.
Oxygen is also directly involved in crucial biological processes like cellular respiration and photosynthesis. During photosynthesis, plants use light energy to convert carbon dioxide and water into glucose and oxygen. Cellular respiration is the process by which organisms use oxygen to break down glucose and release energy, producing carbon dioxide and water as byproducts.
Why Oxygen is Everywhere
Oxygen’s pervasive presence across Earth’s various systems is a direct result of its inherent chemical properties and its abundance. It is the most abundant element in Earth’s crust, making up nearly half of its mass, primarily in the form of various oxides, and constitutes about 89% of the world’s oceans by mass.
Its high reactivity means oxygen readily combines with almost all other elements, forming stable compounds. The continuous cycling of oxygen through processes like photosynthesis and respiration further demonstrates its role in maintaining the balance of Earth’s atmosphere and supporting life.