Chemical compounds represent a substance that consists of two or more bonded elements. The elements bond in a fixed ratio to create a particular compound. The ratio and the types of elements that make it up determine its physical and chemical properties. By combining to form compounds, elements become more stable.
Chemists represent chemical compounds using a variety of formulas that describe the types of elements that make it up. For example, in a molecular formula, the elements that make up the 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 to form "NaCl," which stands 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 compounds.
Chemical compounds differ from simple mixtures of elements due to their physical and chemical properties. The properties of a mixture are more similar to the elements that make up the mixture, but the properties of a compound rarely mimic the contributing elements because, to create a compound, a chemical reaction must take place, resulting in a chemical change. Once a 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 compounds and mixtures.
Similar to elements, chemical compounds can exist in different phases. Solid is the most common, although certain compounds are only in the solid phase at extremely low temperatures. Depending on temperature and pressure, some can exist as liquids, gases, and plasmas as well. As heat is applied, 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. 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 registry dates back to 1957.