Other meanings of Compound
Chemistry
A chemical compound is a substance composed of two or more different chemical elements that are chemically bonded together in a fixed ratio. The elements in a compound are held together by chemical bonds, which can be ionic, covalent, or metallic, and the resulting substance has properties distinct from its constituent elements. For example, sodium (a reactive metal) and chlorine (a toxic gas) combine to form sodium chloride, common table salt, which is stable and edible. Compounds are represented by chemical formulas, such as H2O for water, indicating the types and numbers of atoms present. The concept of a compound is fundamental to chemistry, distinguishing pure substances from mixtures and elements.
A chemical compound is defined by the fixed ratio of its constituent atoms, which is expressed in its chemical formula. This ratio is a consequence of the valency of the elements, which determines how many bonds each atom can form. The bonds that hold atoms together in a compound can be ionic, where electrons are transferred from one atom to another, or covalent, where electrons are shared. Metallic bonds, which involve a 'sea' of delocalized electrons, also give rise to compounds such as alloys, though these are often considered mixtures. The distinction between a compound and a mixture is crucial: in a mixture, the components retain their individual properties and can be separated by physical means, whereas a compound has new properties and requires chemical reactions to break it apart.
The modern concept of a chemical compound emerged from the work of Antoine Lavoisier in the late 18th century, who established the law of conservation of mass and helped systematize chemical nomenclature. Earlier, alchemists had recognized substances like salt and sulfur, but lacked a clear atomic theory. John Dalton's atomic theory (1803) proposed that atoms combine in simple whole-number ratios, providing a theoretical basis for compounds. The development of structural chemistry in the 19th century, notably by August Kekulé and others, led to the understanding of how atoms are arranged in molecules. The discovery of the periodic table by Dmitri Mendeleev in 1869 further organized elements and predicted the existence of yet-unknown compounds.
Compounds are classified into organic and inorganic categories. Organic compounds contain carbon and are often associated with living organisms, while inorganic compounds include salts, oxides, and metals. Within these categories, compounds can be further grouped by functional groups, such as alcohols, acids, and bases. The properties of a compound—such as melting point, boiling point, solubility, and reactivity—are determined by its molecular structure and the types of bonds present. For example, ionic compounds typically have high melting points and conduct electricity when dissolved in water, while covalent compounds often have lower melting points and may be insoluble in water. The study of these properties is essential for applications in medicine, materials science, and industry.
Beyond the common examples, there are many surprising compounds. For instance, helium hydride (HeH+) is the strongest known acid and was detected in space in 2019, though it is unstable on Earth. Another is sodium chloride, which can form a 'superionic' phase under extreme pressure, where chlorine atoms form a lattice and sodium ions flow freely. Some compounds exhibit polymorphism, where the same chemical formula can have different crystal structures, such as carbon in the forms of diamond and graphite. Additionally, the concept of a 'compound' extends to non-stoichiometric compounds, where the ratio of elements can vary slightly, as in iron(II) oxide (FeO) which often has a small excess of oxygen. These examples highlight the complexity and diversity of chemical compounds beyond the textbook definitions.
This article focuses on the chemical sense of the term 'compound'.
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