Oxygen is one of the most abundant elements on Earth, making up about 21% of the atmosphere and nearly half of the Earth’s crust by weight, primarily bound within various minerals. This element is classified as a highly reactive nonmetal, which means it readily combines with most other elements to form new compounds. A chemical reaction involving a substance combining with oxygen is broadly known as oxidation, and the resulting compounds are called oxides. This process is responsible for many everyday phenomena, ranging from the slow decay of metals to the rapid release of energy in combustion.
The Fundamental Chemistry of Oxidation
Oxygen’s strong tendency to react stems from its electron configuration, which leaves it two electrons short of a stable outer shell, similar to a noble gas. This electron “hunger” is quantified by its high electronegativity, a measure of an atom’s ability to attract electrons. Oxygen is the second most electronegative element, surpassed only by fluorine. This characteristic makes oxygen a powerful oxidizing agent, meaning it causes other substances to lose electrons.
When oxygen reacts with another element, the nature of the bond formed depends on the partner element’s own electron-holding ability. If the partner element has a low electronegativity, oxygen will essentially strip electrons away, resulting in an ionic bond. Conversely, if oxygen reacts with an element that has a similarly high electronegativity, they will share electrons to form a covalent bond. In both cases, the partner element is said to be oxidized because it has effectively lost control over its electrons to the more demanding oxygen atom.
Reactions of Metals: Forming Basic Oxides
Metals generally have low electronegativity, making them eager to lose electrons, which aligns perfectly with oxygen’s need to gain them. The reaction between a metal and oxygen typically involves the metal atom transferring its valence electrons to the oxygen atom, forming an ionic compound known as a metal oxide. The ease and speed of this reaction vary widely depending on the specific metal’s position on the periodic table.
Highly reactive metals, such as the alkali metals (Group 1 like sodium and potassium) and alkaline earth metals (Group 2 like magnesium and calcium), react spontaneously and often vigorously with oxygen. For instance, sodium reacts so quickly that it must be stored under oil to prevent immediate oxidation upon exposure to air. Magnesium, while slightly less reactive than sodium, burns with an intense, bright flame when ignited, forming magnesium oxide. These reactions are examples of rapid oxidation, often characterized as combustion.
Transition metals, such as iron and copper, show a much slower reaction rate under normal atmospheric conditions. The familiar process of iron rusting is a form of slow oxidation, where iron reacts with oxygen and moisture to form iron oxide (rust) over an extended period. The resulting metal oxides are characterized as basic oxides because they react with water to form hydroxides, which are bases. For example, sodium oxide reacts with water to produce sodium hydroxide, a strong base.
Reactions of Nonmetals and Metalloids
The reaction of oxygen with nonmetals involves a different bonding mechanism compared to metals. Since nonmetals also have relatively high electronegativities, they tend to share electrons with oxygen rather than fully transfer them, resulting in the formation of covalent bonds. The compounds formed are nonmetal oxides, which exhibit varied chemical properties and structures. Carbon, for instance, can react with oxygen to form carbon monoxide (\(\text{CO}\)) when oxygen supply is limited, or carbon dioxide (\(\text{CO}_2\)) when oxygen is plentiful.
Nonmetal oxides are generally classified as acidic oxides because they react with water to produce acidic solutions. Sulfur, a common nonmetal, reacts with oxygen to form sulfur dioxide (\(\text{SO}_2\)), which then dissolves in water to create sulfurous acid. Similarly, nitrogen oxides, such as nitrogen dioxide (\(\text{NO}_2\)), are formed, and they contribute to the formation of nitric acid in the atmosphere. This acidic nature is a significant factor in environmental issues like acid rain.
Some nonmetal oxides, like carbon monoxide (\(\text{CO}\)) and nitrous oxide (\(\text{N}_2\text{O}\)), are considered neutral oxides as they do not react with acids or bases. Metalloids, elements like silicon that possess properties of both metals and nonmetals, also react with oxygen to form oxides, such as silicon dioxide (\(\text{SiO}_2\)), which is the primary component of sand and quartz. Finally, the noble gases (Group 18), such as neon and argon, are the only elements that do not readily react with oxygen due to their completely filled outer electron shells, making them chemically inert.