Chemical compounds represent a substance that consists of two or more bonded elements. The elements bond in a fixed ratio to create a particular chemical compound. The ratio and the types of elements determine the physical and chemical properties for the compound. By combining to form compounds, elements become more stable.

Chemists represent chemical compounds using a variety of formulas that describes the types of elements in the compound. For example, in a molecular formula, the elements that make up the chemical compound are listed, including the ratio in which each element appears. "Na," which is the elemental symbol for sodium, and "Cl," which is the elemental symbol for chlorine, combine for the chemical compound "NaCl," which is short for sodium chloride, or table salt.

More complex compounds require a more complex formula. For example, water is represented by H₂O, meaning two parts hydrogen per one part oxygen. Other formulas reveal some of the structural information for the chemical compounds.

Chemical compounds differ from simple mixtures of elements by the physical and chemical properties of the compounds. For example, the properties of a mixture are more similar to the elements that make up the mixture. The properties of a compound, however, rarely mimic the contributing elements because, to create a compound, a chemical reaction must take place, resulting in a chemical change. Once a chemical compound is created, breaking it down often requires the addition of energy, such as heat, while breaking down mixtures often only requires a simple filtering process. Some substances exhibit the properties of both chemical compounds and mixtures.

Similar to elements, chemical compounds can exist in different phases. Solid is the most common phase for a chemical compound, although certain compounds are only in the solid phase at extremely low temperatures. Depending on temperature and pressure, some compounds can exist as liquids, gases, and plasmas. As heat is applied to chemical compounds, they typically begin decomposing into smaller compounds or individual elements.

Each chemical compound is tracked by the American Chemical Society's Chemical Abstracts Service, sometimes called the CAS. Chemical compounds receive a unique identifying number, called a CAS registry number. The CAS tracks tens of millions of unique substances, and thousands of new ones are added daily. Development of the CAS registry dates back to 1957.