Chemical compound

A chemical compound is a substance made of two or more different types of atoms. These atoms are called chemical elements. In a compound, these elements are held together in a fixed, specific proportion. This means the ratio of one element to another stays the same. For example, water is always made of two hydrogen atoms and one oxygen atom. We write this as H2O. If a group of atoms only contains one type of element, it is not a compound.

Chemical compounds are held together by chemical bonds. These bonds are the forces that connect atoms in a specific spatial arrangement. One common type is the covalent bond, also called a molecular bond. This happens when atoms share electrons to reach a stable state. Another type is the ionic bond. This occurs when one atom transfers electrons to another atom. This transfer creates oppositely charged ions. The positive ions are called cations, and the negative ions are called anions. These opposite charges pull the atoms together through electrostatic forces.

Scientists classify compounds into four major types based on their bonding. Molecular compounds use covalent bonds to hold atoms together. Ionic compounds, often called salts, use ionic bonds. These often form continuous three-dimensional networks rather than individual molecules. Intermetallic compounds are a type of metallic alloy. They form an ordered solid-state structure between two or more metallic elements. Finally, coordination complexes use coordinate covalent bonds. These consist of a central metal atom or ion, called a coordination centre. This centre is surrounded by molecules or ions known as ligands.

The study of these substances has a long history. In 1661, Robert Boyle published a book called "The Sceptical Chymist." He used terms like "compounded body" and "perfectly mixt body." Boyle also used the idea of "corpuscles" or "atomes" to explain how elements combine. Later, in 1724, the English minister Isaac Watts provided a clearer definition. He helped distinguish between a chemical element and a chemical compound using modern terms.

Chemical reactions allow compounds to transform into new substances. During a reaction, a compound interacts with another substance. This process involves breaking existing chemical bonds between atoms. Once the old bonds are broken, new bonds form between different atoms. This creates a completely different chemical composition. This mechanism allows for the vast variety of materials found in our universe.

There are many ways to identify and measure compounds. Scientists use a chemical formula to show the number of atoms in a molecule. They use standard chemical symbols and numerical subscripts to do this. Many compounds also have a unique CAS number. This identifier is assigned by the Chemical Abstracts Service. Today, more than 350,000 chemical compounds and mixtures are registered for use globally.

Some substances are more complex than simple compounds. Non-stoichiometric compounds are a disputed or marginal case. In these substances, the proportions of elements are not simple whole numbers. For example, palladium hydride is written as PdHx, where x is between 0.02 and 0.58. Many silicate minerals in the Earth's crust and mantle are also non-stoichiometric. These may contain foreign elements trapped in their crystal structures. These substances are related to, but sometimes distinct from, true chemical compounds.