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The diversity of oxides: Why can the same element form many different oxides?
An oxide is a chemical compound that contains at least one oxygen atom and chemical formulas of other elements. In the Earth's crust, most of it is composed of oxides. Even some materials that are considered pure elements often form oxides. For example, aluminum foil forms a thin layer of aluminum oxide (Al2O3) on its surface, called a passivation layer, which protects it from further oxidation.
The diversity of oxides is reflected in their different chemical compositions and structures.
The chemical composition and structure of oxides vary, which allows the same element to form many different oxides. A famous example is the oxides of carbon. Carbon dioxide (CO2) and carbon monoxide (CO) show the oxides of carbon in different oxidation states. These binary oxides are just the tip of the iceberg of oxide diversity. In fact, there are more complex oxides, and the formation of these oxides often involves the intervention of other cations or anions.
Formation of oxides
Except for a few noble gases, most elements can form oxides. There are also various pathways for the formation of oxides, especially metal oxides. Many metal oxides are derived from the decomposition of other metal compounds, such as carbonates, hydroxides, and nitrates. In the process of making calcium oxide, calcium carbonate decomposes when heated to release carbon dioxide.
CaCO3 ⟶ CaO + CO2
Almost all elements form oxides when heated in the presence of oxygen. For example, zinc powder burns in air to form zinc oxide:
2 Zn + O2 ⟶ 2 ZnO
The extraction of metal ores usually involves burning the metal sulfide ores in air to produce oxides. For example, molybdenum disulfide (MoS2) is converted into molybdenum trioxide in this process, which is the precursor of all molybdenum compounds.
2 MoS2 + 7 O2 ⟶ 2 MoO3 + 4 SO2
Metal and non-metal oxides
Among non-metal oxides, carbon dioxide and carbon monoxide are very important and common oxides. These oxides are the products of complete or partial oxidation of carbon or hydrocarbons. In the absence of oxygen, carbon monoxide is produced, while excess oxygen produces carbon dioxide.
CH4 + 3/2 O2 ⟶ CO + 2 H2O
CH4 + 2 O2 ⟶ CO2 + 2 H2O
Also, the process of converting nitrogen into oxides is more difficult, but when ammonia is burned it produces nitric oxide, which further reacts with oxygen to produce nitrogen dioxide:
4 NH3 + 5 O2 ⟶ 4 NO + 6 H2O
NO + 1/2 O2 ⟶ NO2
Structure and reaction of oxides
The structures of oxides vary, from single molecules to polymers and crystalline structures. Solid metal oxides often form polymer structures under ambient conditions. While most metal oxides are crystalline solids, many nonmetal oxides are gaseous molecules, such as carbon dioxide and carbon monoxide.
The reduction reaction of metal oxides has been widely used in the production of some metals. Metal oxides are often reduced by heating, while some oxides require chemical reduction, with carbon being a common reducing agent.
2 Fe2O3 + 3 C ⟶ 4 Fe + 3 CO2
In addition, the dissolution of oxides is also related to strong M-O bonds. Although metal oxides are usually insoluble in solvents, they can be attacked and produce oxygen anions in the presence of acids and bases.
Nomenclature and formula of oxides
The chemical formula of an oxide can usually be derived from the element's highest oxidation state. The predictability of these formulas allows us to understand the formation of a wide variety of oxides, even tetraoxygens like O4.
The formation of O4 also reflects the regularity of oxides.
These diverse oxides provide us with rich chemical knowledge and reveal many mysteries of nature. In the future, with the advancement of science and technology, we may be able to discover more properties of oxides and even explore their potential applications. How many ways and forms of oxides are there that we don't know about?
