Oxide

An oxide is made of air and one other thing.

Most of the ground is made of them. They can be hard solids or light gases.

Some metals change when they touch air. This can make a thin skin on the metal. This skin can help protect it. Other metals like gold do not change at all. They do not join with the air. Oxides are found all around us in the world.

An oxide is a compound made of oxygen and one other element.

Most of the Earth's crust is made of oxides. They can be gases or hard solids.

Some oxides are very simple. We call these binary oxides. They have only two parts. For example, carbon monoxide and carbon dioxide are binary oxides. Other oxides are more complex. They can have many different parts.

Oxides form in many ways. Some metal oxides form when you heat them. For instance, zinc powder burns in air to make zinc oxide. Other oxides come from breaking down old materials. Heating limestone makes calcium oxide. It lets out carbon dioxide in the process.

Many metals react with oxygen in the air. This can cause corrosion. Some metals like aluminum grow a thin skin. This is called a passivation layer. It helps protect the metal. Other metals like gold do not react with oxygen.

We also use oxides to make metals. We can use heat to break them down. This is called thermal decomposition. Sometimes we use carbon to help. This helps make iron from ore.

Caption: This shows the parts of a titanium oxide.

An oxide is a chemical compound. It must contain at least one oxygen atom and one other element. You can find these compounds almost everywhere. In fact, most of the Earth's crust is made of oxides. Even materials that look like pure elements often have an oxide coating. For example, aluminum foil grows a thin skin called a passivation layer. This layer acts like a shield. It protects the foil from further oxidation.

Oxides come in many different shapes and sizes. Some are very simple and are called binary oxides. These contain only oxygen and one other element. A good example is carbon monoxide or carbon dioxide. Other oxides are much more complex. They can include many different parts, like extra ions. The mineral fayalite is a ternary oxide. This means it has three parts, such as iron, silicon, and oxygen. Some metal oxides can even have different structures. Titanium dioxide is a famous example because it has three distinct structures.

Many different ways can create an oxide. Some metal oxides form when you heat other compounds. For instance, heating limestone breaks it down into calcium oxide. This process also releases carbon dioxide gas. Other metals form oxides by reacting with oxygen in the air. This can lead to corrosion in iron. Sometimes, workers use a process called roasting to make oxides. They heat metal sulfide minerals in the air. This turns molybdenite into molybdenum trioxide. This trioxide is used to make almost all molybdenum compounds.

We can also use oxides to get pure metals. This is often done through a process called reduction. One way is thermal decomposition, which means using heat to break a compound apart. For example, silver oxide decomposes at 200 °C. Another way uses a chemical reagent. Carbon in the form of coke is a common and cheap reducing agent. This is how people smelt iron ore. Other non-metal oxides are very common in our world. Carbon dioxide and carbon monoxide form when carbon reacts with oxygen. Nitrogen can also form oxides, like nitric oxide, through combustion.

Oxides can be found in many states of matter. Some are gases, like carbon dioxide. Others are crystalline solids, like many metal oxides. Some oxides can even dissolve in liquids. Adding an aqueous base to certain oxides can create oxyanions. This is a way of describing how they change in water. You might know about sulfuric acid, which is made on a huge scale. It starts by oxidizing sulfur to sulfur dioxide. Then, it is oxidized again to become sulfur trioxide. Finally, a hydration reaction turns it into sulfuric acid.

An oxide is a chemical compound containing at least one oxygen atom and at least one other element. The term "oxide" specifically refers to the dianion, which is an O2− ion. This ion carries a net charge of −2 because the oxygen is in an oxidation state of −2. Oxides are incredibly important because they make up most of the Earth's crust. Even substances that appear to be pure elements often possess a thin oxide coating. For example, aluminum foil develops a passivation layer. This is a thin skin of aluminum oxide that protects the underlying metal from further oxidation.

Oxides vary greatly in their stoichiometry, which is the measurable relationship between the elements in a chemical equation. Binary oxides are the simplest type because they contain only oxygen and one other element. Common examples include carbon monoxide and carbon dioxide. Many oxides are more complex than binary versions. Complexity can arise when other cations, or positively charged ions, are introduced. Other anions, or negatively charged ions, can also be added to the structure. A ternary oxide contains three different parts. The mineral fayalite, Fe2SiO4, is a well-known example of a ternary oxide. Some metal oxides also show polymorphism, meaning they can exist in different structures. For instance, the commercially important titanium dioxide exists in three distinct structures.

The formation of oxides happens through many different chemical pathways. Metal oxides often arise from the decomposition of other metal compounds like carbonates, hydroxides, or nitrates. When you heat calcium carbonate, also known as limestone, it breaks down into calcium oxide. This process also releases carbon dioxide gas. Many elements also form oxides when they react with oxygen in the air. This reaction is a key part of corrosion, especially in iron. In industrial settings, workers use a process called roasting to produce oxides. Roasting involves heating metal sulfide minerals in the air. This process converts molybdenite into molybdenum trioxide, which is the precursor for almost all molybdenum compounds.

Non-metal oxides are also very prevalent in our world. Carbon monoxide and carbon dioxide form through the oxidation of carbon or hydrocarbons. If there is a deficiency of oxygen, carbon monoxide is produced. If there is an excess of oxygen, the process moves through carbon monoxide to create carbon dioxide. Nitrogen is more difficult to convert into oxides. However, the combustion of ammonia can produce nitric oxide. This can then react with oxygen to create more oxides. These reactions are used to produce nitric acid. Similarly, sulfuric acid is produced on a massive industrial scale. This begins by oxidizing sulfur to sulfur dioxide, then oxidizing that to sulfur trioxide. Finally, a hydration reaction turns the trioxide into sulfuric acid.

The physical structure of an oxide depends on its composition. Oxides can exist as individual molecules, polymeric structures, or crystalline structures. At standard conditions, they can be solids or gases. Most non-metal oxides, such as carbon dioxide and carbon monoxide, are molecular. All simple oxides of nitrogen, like NO and NO2, are also molecular. Some molecular oxides are more complex, such as phosphorus pentoxide, which has the formula P4O10. Rare tetroxides also exist, including ruthenium tetroxide and osmium tetroxide. In contrast, most metal oxides are crystalline solids that often have polymeric structures at ambient conditions.

Oxides can also be used to recover pure metals through a process called reduction. One method is thermal decomposition, where heat breaks the compound apart. For example, silver oxide decomposes at 200 °C. A more common method uses a chemical reagent. Carbon in the form of coke is a very common and cheap reducing agent. This is the primary method used in iron ore smelting. Some metal oxides also undergo reductive dissolution. This is a process where oxides dissolve in the presence of reducing agents, such as organic compounds. This specific phenomenon is an integral part of the geochemical iron cycle.

Finally, the chemical behavior of oxides includes hydrolysis and dissolution. Because the bonds between metals and oxygen are typically strong, many metal oxides do not dissolve easily in solvents. However, they can be attacked by aqueous acids or bases. When oxides dissolve, they often produce oxyanions. For example, adding an aqueous base to certain compounds can create various phosphates or polyoxometalates. Some metal oxides can also react with an alkaline solution of hydrogen peroxide to create metal peroxide compounds. The chemical formulas for oxides in their highest oxidation state are often predictable based on the number of valence electrons. One notable exception is copper, where the highest oxidation state oxide is copper(II) oxide rather than copper(I) oxide.