Chemical compound
A chemical compound is two hydrogen atoms bonded to one oxygen atom, and that arrangement gives us water, with the formula H2O. That same logic of bonded elements scales almost without limit. Globally, more than 350,000 chemical compounds, including mixtures of chemicals, have been registered for production and use. The defining rule is strict. A compound contains atoms from more than one chemical element, held together by chemical bonds. A molecule made of atoms from only one element is therefore not a compound at all. How did thinkers first draw that line between an element and a compound? What holds these substances together, and what happens when one compound meets another? The answers run from a chemist writing in 1661 to the temporary flickers of force between adjacent atoms.
Robert Boyle's The Sceptical Chymist, published in 1661, is among the earliest uses of the term compound in a sense close to the modern one. Boyle reached for several phrases interchangeably. He wrote of a compound, a compounded body, a perfectly mixt body, and a concrete. His perfectly mixt bodies included gold, lead, mercury, and wine, a list that blurs the line between compound and mixture. Yet the distinction Boyle cared about most was the one between element and compound, and that became a central theme of his work.
Quicksilver, Boyle observed, could be transformed and recovered again. Treated with aqua fortis it became a white powder, and combined with sulphur it formed a blood-red and volatile cinnabar. From all these exotic compounds, he noted, the very same running mercury could be recovered. To explain how a small set of elements could yield so many substances, Boyle used the idea of corpuscles, which he also called atomes. If each element's corpuscles had a peculiar size and shape, he reasoned, they could mingle in various proportions and connect in many ways. From that, he wrote, an almost incredible number of concretes might be composed.
Isaac Watts, the English minister and logician, sharpened the definition in his Logick, published in 1724. Watts called some substances simple and some compound. Simple substances, usually called elements, were those that cannot be resolved or reduced into two or more substances of different kinds. He dismissed the older Aristotelian scheme of fire, air, earth, and water, plus a fifth heavenly quintessence, as a doctrine since refuted by experimental philosophy. Watts illustrated his point with a craftsman's eye. A needle, made only of steel, is a simple body. A sword or a knife is a compound, because its handle is made of materials different from the blade.
Any substance consisting of two or more different types of atoms in a fixed stoichiometric proportion can be termed a chemical compound. That fixed proportion is the heart of the definition. A chemical formula expresses it, using chemical symbols for the elements and subscripts to indicate the number of atoms involved. Pure chemical elements generally fail this test, since they do not meet the two-or-more-element requirement, even when they form molecules like the diatomic H2 or the polyatomic S8. Many compounds also carry a unique numerical identifier from the Chemical Abstracts Service, known as a CAS number.
Non-stoichiometric compounds complicate that clean picture and form a disputed marginal case. Their proportions can be reproducible in preparation and yield fixed proportions of their component elements, yet those proportions are not integral. Palladium hydride is the example given, written PdHx, where x falls between 0.02 and 0.58. The nomenclature here is varying and sometimes inconsistent, separating these substances from true compounds that require fixed ratios.
Many silicate minerals show the same wrinkle. They are chemical substances without simple formulae reflecting bonding in fixed ratios, yet they are often called non-stoichiometric compounds. Their variable compositions can come from foreign elements trapped in the crystal structure, or from an excess or deficit of constituent elements at places in the structure. Such non-stoichiometric substances form most of the crust and mantle of the Earth. Even compounds regarded as chemically identical can differ slightly, when varying amounts of heavy or light isotopes change the ratio of elements by mass.
Four major types of compound are distinguished by how their constituent atoms are bonded together. Molecular compounds are held together by covalent bonds, while ionic compounds are held by ionic bonds. Intermetallic compounds rely on metallic bonds, and coordination complexes are held together by coordinate covalent bonds. A molecule itself is an electrically neutral group of two or more atoms. It may be homonuclear, like the oxygen molecule O2, or heteronuclear, like water with its two hydrogen atoms and one oxygen atom. A molecule is the smallest unit of a substance that still carries all its physical and chemical properties.
An ionic compound is composed of ions held together by electrostatic forces, neutral overall but built from positively charged cations and negatively charged anions. These can be simple, like the sodium and chloride ions in sodium chloride, or polyatomic, like the ammonium and carbonate ions in ammonium carbonate. Individual ions usually have multiple nearest neighbours, so rather than forming molecules they form a continuous three-dimensional network, usually crystalline. Ionic compounds containing basic ions such as hydroxide or oxide are classified as bases, while those without are known as salts. Salts can form through acid-base reactions, evaporation of solvent, precipitation, freezing, or the reaction of reactive metals with reactive non-metals. As solids these compounds are typically hard, brittle, and electrically insulating, but once melted or dissolved they conduct well, because the ions are mobilized.
An intermetallic compound is a metallic alloy that forms an ordered solid-state compound between two or more metallic elements. Intermetallics are generally hard and brittle, with good high-temperature mechanical properties, and can be classified as stoichiometric or nonstoichiometric. A coordination complex, by contrast, centres on a single atom or ion, usually metallic, called the coordination centre. Around it sits an array of bound molecules or ions known as ligands or complexing agents. Many compounds of transition metals are coordination complexes of exactly this kind.
London dispersion forces are the weakest of all intermolecular forces and form part of the Van der Waals force. They are temporary attractions, appearing when the electrons in two adjacent atoms line up to create a momentary dipole. These fleeting forces can produce Van der Waals molecules. They are also responsible for condensing nonpolar substances into liquids, and for freezing them further into solids when the temperature drops low enough.
A covalent bond, also called a molecular bond, involves two atoms sharing electrons. It tends to occur between elements close to each other on the periodic table, though it appears between some metals and nonmetals too. Elements near one another tend to have similar electronegativities, meaning a similar affinity for electrons. With neither atom strongly inclined to donate or gain, the two share electrons so each reaches a more stable octet.
Ionic bonding works the opposite way, with valence electrons completely transferred between elements. A metal usually loses its valence electrons to become a positively charged cation, while a nonmetal gains them to become a negatively charged anion. The bond forms between an electron donor, usually a metal, and an electron acceptor, usually a nonmetal. Hydrogen bonding follows yet another pattern. A hydrogen atom bonded to an electronegative atom forms an electrostatic connection with another electronegative atom, through interacting dipoles or charges.
A compound can be transformed into a different substance through a chemical reaction, often by interaction with a second compound. In that process, bonds between atoms break in both interacting compounds, and then reform so that new associations are made between atoms. The pattern can be written schematically as AB plus CD yields AD plus CB. Here A, B, C, and D are each unique atoms, while AB, AD, CD, and CB are each unique compounds. That simple swap of partners is how one defined chemical structure, held in a defined spatial arrangement, becomes another, and it is the reason a fixed set of elements can keep producing new substances without end.
Common questions
What is a chemical compound?
A chemical compound is a chemical substance composed of many identical molecules containing atoms from more than one chemical element, held together by chemical bonds. A molecule made of atoms from only one element is not a compound. Compounds contain two or more elements in a fixed stoichiometric proportion.
What are the four major types of chemical compound?
The four major types of chemical compound are molecular compounds held together by covalent bonds, ionic compounds held together by ionic bonds, intermetallic compounds held together by metallic bonds, and coordination complexes held together by coordinate covalent bonds. Non-stoichiometric compounds form a disputed marginal case.
Who first used the term compound in a modern sense?
Robert Boyle used the term compound in a sense similar to the modern one at least as early as 1661, when his book The Sceptical Chymist was published. He also used the terms compounded body, perfectly mixt body, and concrete. In 1724, Isaac Watts gave an early definition of chemical element in his Logick that contrasted element with compound in clear, modern terms.
How is a chemical compound written as a formula?
A chemical formula specifies the number of atoms of each element in a compound, using standard chemical symbols with numerical subscripts. For example, water is composed of two hydrogen atoms bonded to one oxygen atom, written as H2O. Many compounds also carry a unique CAS number assigned by the Chemical Abstracts Service.
How many chemical compounds have been registered for use?
Globally, more than 350,000 chemical compounds, including mixtures of chemicals, have been registered for production and use. Each can be transformed into a different substance through a chemical reaction that breaks and reforms bonds between atoms.
What is a non-stoichiometric compound?
A non-stoichiometric compound has reproducible proportions of its component elements that are not integral, such as palladium hydride written PdHx where x falls between 0.02 and 0.58. Many silicate minerals are called non-stoichiometric compounds, and such substances form most of the crust and mantle of the Earth.