Chemical compound

A **chemical compound** is a substance made of two or more different elements joined together by chemical bonds in a fixed ratio. Many compounds are made of separate molecules. Each molecule has the same number and types of atoms. For example, every water molecule has two hydrogen atoms and one oxygen atom. Other compounds do not have separate molecules. Instead, their atoms are arranged in a large, repeating network. For example, sodium chloride (table salt) is made of equal numbers of sodium and chlorine atoms, but they are all connected in a big crystal structure.

One important part of this definition is that the elements must be different. That means something like oxygen gas (O₂) or nitrogen gas (N₂) is not a compound. These are molecules made of only one type of element, so they are not compounds. Chemical compounds can be made or broken apart during chemical reactions. In these reactions, atoms are rearranged to form new compounds. For example, when hydrogen and oxygen gases react, they combine to make water. One early written use of the word "compound" was by Robert Boyle in his 1661 book "The Sceptical Chymist". He talked about substances made of more than one component. However, the modern definition of a chemical compound appeared in the late 18th and early 19th centuries with scientists like Antoine Lavoisier and John Dalton.

Chemical compounds have something called emergent properties. This means they act differently from the elements they are made of. When elements join together to form a compound, they create a new substance with its own special properties.These new properties cannot be found in the elements that make them up. This happens because the atoms are connected in a new way. The way they bond and arrange themselves changes how the compound looks and behaves. The compound might be a solid, liquid, or gas. It might have a new taste, smell, or react differently with other substances. For example, table salt is a compound called sodium chloride. It is made from sodium, a soft metal that reacts strongly with water, and chlorine, a poisonous green gas. But when they combine, they form a safe, white crystal that we use to flavor food. Another example is water. It is made from two gases, hydrogen and oxygen. On their own, they are both gases, but when they bond together, they form a liquid that all living things need to survive.

Chemical compounds can be found in all three states of matter. Some are solids, like sodium chloride (table salt). Others are liquids, like water. While others are gases, like carbon dioxide. Compounds can come from natural sources, such as plants, animals, or minerals. For example, sugar comes from sugarcane or sugar beets, and oxygen is made by plants during photosynthesis. Scientists can also create compounds in laboratories. These are called synthetic chemicals. Some synthetic compounds are made to copy natural ones, while others are made for special uses. For example, medicines, plastics, or cleaning products. Whether natural or synthetic, all compounds follow the same rules of chemistry.

Chemical compounds are different from both mixtures and elements. Compounds are substances made when two or more different elements join together through chemical bonds. This gives compounds their own fixed structure and unique properties. A mixture is not the same. It is a physical combination of two or more substances. In a mixture, the parts are not chemically joined, so they can be in any amount. In a mixture, there are no new properties made when the two substances are mixed together. Mixtures can usually be separated easily using physical methods like filtering, evaporating, or sorting. For example, salt water is a mixture. You can add more salt to salt water, and it would still be salt water. You cannot add more oxygen to water (H₂O). If you do, it is no longer water but hydrogen peroxide (H₂O₂). You can get the salt back by evaporating the water. No chemical reaction is needed. An element is a pure substance made of only one kind of atom. Elements are the basic building blocks of all matter. You cannot break them down into anything simpler by chemical means. For example, gold is an element made only of gold atoms. You cannot break it down into smaller parts through a chemical reaction.

Chemical compounds can be grouped based on the type of bond that holds their atoms together. A covalent compound is formed when atoms share electrons forming a covalent bond. These compounds usually form between nonmetal elements. Water (H₂O) and carbon dioxide (CO₂) are examples. An ionic compound is formed when one atom gives up electrons to another, creating charged particles called ions. These compounds usually form between metals and nonmetals. The positive and negative ions stick together because of their opposite charges forming an ionic bond. A common example is sodium chloride (NaCl), or table salt. Coordination complexes are compounds where a central metal atom is surrounded by other atoms or molecules called ligands. They are bonded together by coordinate covalent bonds. In this type of bond, both of the shared electrons come from the same atom. Hemoglobin in blood is a coordination complex with iron at its center. A non-stoichiometric compound does not have exact whole-number ratios between its elements. These compounds often form in solid crystals, especially in metal oxides.

Chemical compounds can be grouped into two main types: organic and inorganic compounds. An organic compound is any compound that contains carbon atoms bonded with hydrogen atoms, and often with oxygen, nitrogen, or other elements. These compounds are the basis of life and include things like sugars, fats, proteins, and fuels like gasoline. Inorganic compounds, on the other hand, are compounds that do not contain carbon-hydrogen (C-H) bonds. They include salts, metals, minerals, and many acids and bases. Water (H₂O), table salt (NaCl), and carbon dioxide (CO₂) are examples of inorganic compounds. The idea of organic compounds started in the 1800s. At first, scientists believed that organic compounds could only be made by living things. This changed in 1828, when a chemist named Friedrich Wöhler made urea, an organic compound, from a non-living chemical. This discovery showed that organic compounds could be made in the lab too, not just by nature.

A chemical formula is a way to show what elements are in a chemical compound and how many atoms of each element are present. It uses the symbols of the elements from the periodic table and numbers to show what is in the compound. For example, the formula H₂O means a water molecule has 2 hydrogen atoms and 1 oxygen atom. To make sure scientists all over the world use the same names and formulas, an organization called IUPAC (International Union of Pure and Applied Chemistry) creates rules for naming compounds. This system is called chemical nomenclature. It helps chemists write names and formulas that clearly show what a compound is, how it is built, and how it might react. In addition to a name and formula, many chemical compounds also have a unique number called a CAS number. This number is assigned by the Chemical Abstracts Service. No matter what name or formula is used in different countries or languages, the CAS number always refers to the exact same substance. This helps avoid confusion in science, industry, and research.