Substance composed of multiple chemically bonded elements
Purewater (H2O) is an example of a compound. Theball-and-stick model of the molecule shows the spatial association of two partshydrogen (white) and one partoxygen (red)
Achemical formula specifies the number of atoms of each element in a compound molecule, using the standardchemical symbols with numericalsubscripts. Many chemical compounds have a uniqueCAS number identifier assigned by theChemical Abstracts Service. Globally, more than 350,000 chemical compounds (including mixtures of chemicals) have been registered for production and use.[1]
History of the concept
Robert Boyle
Robert BoyleTitle page ofThe Sceptical Chymist
The term "compound"—with a meaning similar to the modern—has been used at least since 1661 whenRobert Boyle'sThe Sceptical Chymist was published. In this book, Boyle variously used the terms "compound",[2] "compounded body",[3] "perfectly mixt body",[4] and "concrete".[5] "Perfectly mixt bodies" included for example gold,[4] lead,[4] mercury,[2] and wine.[6] While the distinction between compound andmixture is not so clear, the distinction betweenelement and compound is a central theme.
Quicksilver ... withAqua fortis will be brought into a ... white Powder ... with Sulphur it will compose a blood-red and volatile Cinaber. And yet out of all these exotick Compounds, we may recover the very same running Mercury.[7]
Corpuscles of elements and compounds
Boyle used the concept of "corpuscles"—or "atomes",[8] as he also called them—to explain how a limited number of elements could combine into a vast number of compounds:
If we assigne to the Corpuscles, whereof each Element consists, a peculiar size and shape ... such ... Corpuscles may be mingled in such various Proportions, and ... connected so many ... wayes, that an almost incredible number of ... Concretes may be compos’d of them.[5]
In hisLogick, published in 1724, the English minister and logicianIsaac Watts gave an early definition of chemical element, and contrasted element with chemical compound in clear, modern terms.[9]
Among Substances, some are called Simple, some are Compound ... Simple Substances ... are usually called Elements, of which all other Bodies are compounded: Elements are such Substances as cannot be resolved, or reduced, into two or more Substances of different Kinds. ... Followers of Aristotle made Fire, Air, Earth and Water to be the four Elements, of which all earthly Things were compounded; and they suppos'd the Heavens to be a Quintessence, or fifth sort of Body, distinct from all these : But, since experimental Philosophy ... have been better understood, this Doctrine has been abundantly refuted. The Chymists make Spirit, Salt, Sulphur, Water and Earth to be their five Elements, because they can reduce all terrestrial Things to these five : This seems to come nearer the Truth ; tho' they are not all agreed ... Compound Substances are made up of two or more simple Substances ... So a Needle is simple Body, being made only of Steel; but a Sword or a Knife is a compound because its ... Handle is made of Materials different from the Blade.
Definitions
Any substance consisting of two or more different types ofatoms (chemical elements) in a fixedstoichiometric proportion can be termed achemical compound; the concept is most readily understood when considering purechemical substances.[10]: 15 [11][12] It follows from their being composed of fixed proportions of two or more types of atoms that chemical compounds can be converted, viachemical reaction, into compounds or substances each having fewer atoms.[13] Achemical formula is a way of expressing information about the proportions of atoms that constitute a particular chemical compound, usingchemical symbols for the chemical elements, andsubscripts to indicate the number of atoms involved. For example,water is composed of twohydrogen atoms bonded to oneoxygen atom: the chemical formula is H2O. In the case ofnon-stoichiometric compounds, the proportions may be reproducible with regard to their preparation, and give fixed proportions of their component elements, but proportions that are not integral [e.g., forpalladium hydride, PdHx (0.02 < x < 0.58)].[14]
There is varying and sometimes inconsistent nomenclature differentiating substances, which include truly non-stoichiometric examples, from chemical compounds, which require the fixed ratios. Many solid chemical substances—for example manysilicate minerals—are chemical substances, but do not have simple formulae reflecting chemically bonding of elements to one another in fixed ratios; even so, thesecrystalline substances are often called "non-stoichiometric compounds". It may be argued that they are related to, rather than being chemical compounds, insofar as the variability in their compositions is often due to either the presence of foreign elements trapped within the crystal structure of an otherwise known truechemical compound, or due to perturbations in structure relative to the known compound that arise because of an excess of deficit of the constituent elements at places in its structure; such non-stoichiometric substances form most of thecrust andmantle of the Earth. Other compounds regarded as chemically identical may have varying amounts of heavy or lightisotopes of the constituent elements, which changes the ratio of elements by mass slightly.
A molecule is anelectrically neutral group of two or more atoms held together by chemical bonds.[16][17][18] A molecule may behomonuclear, that is, it consists of atoms of one chemical element, as with two atoms in theoxygen molecule (O2); or it may beheteronuclear, a chemical compound composed of more than one element, as withwater (two hydrogen atoms and one oxygen atom; H2O). A molecule is the smallest unit of a substance that still carries all the physical and chemical properties of that substance.[19]
An ionic compound is a chemical compound composed ofions held together byelectrostatic forces termedionic bonding. The compound is neutral overall, but consists of positively charged ions calledcations and negatively charged ions calledanions. These can besimple ions such as thesodium (Na+) andchloride (Cl−) insodium chloride, orpolyatomic species such as theammonium (NH+ 4) andcarbonate (CO2− 3) ions inammonium carbonate. Individual ions within an ionic compound usually have multiple nearest neighbours, so are not considered to be part of molecules, but instead part of a continuous three-dimensional network, usually in acrystalline structure.
An intermetallic compound is a type ofmetallicalloy 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.[20][21][22] They can be classified as stoichiometric or nonstoichiometric intermetallic compounds.[20]
A coordination complex consists of a central atom or ion, which is usuallymetallic and is called thecoordination centre, and a surrounding array of bound molecules or ions, that are in turn known asligands or complexing agents.[23][24][25] Many metal-containing compounds, especially those oftransition metals, are coordination complexes.[26] A coordination complex whose centre is a metal atom is called a metal complex of d block element.
Bonding and forces
Compounds are held together through a variety of different types of bonding and forces. The differences in the types of bonds in compounds differ based on the types of elements present in the compound.
London dispersion forces are the weakest force of allintermolecular forces. They are temporary attractive forces that form when theelectrons in two adjacent atoms are positioned so that they create a temporarydipole. Additionally, London dispersion forces are responsible for condensingnon polar substances to liquids, and to further freeze to a solid state dependent on how low the temperature of the environment is.[27]
Acovalent bond, also known as a molecular bond, involves the sharing of electrons between two atoms. Primarily, this type of bond occurs between elements that fall close to each other on theperiodic table of elements, yet it is observed between some metals and nonmetals. This is due to the mechanism of this type of bond. Elements that fall close to each other on the periodic table tend to have similarelectronegativities, which means they have a similar affinity for electrons. Since neither element has a stronger affinity to donate or gain electrons, it causes the elements to share electrons so both elements have a more stableoctet.
Ionic bonding occurs whenvalence electrons are completely transferred between elements. Opposite to covalent bonding, this chemical bond creates two oppositely charged ions. The metals in ionic bonding usually lose their valence electrons, becoming a positively chargedcation. The nonmetal will gain the electrons from the metal, making the nonmetal a negatively chargedanion. As outlined, ionic bonds occur between an electron donor, usually a metal, and an electron acceptor, which tends to be a nonmetal.[28]
A compound can be converted to a different chemical composition by interaction with a second chemical compound via achemical reaction. In this process, bonds between atoms are broken in both of the interacting compounds, and then bonds are reformed so that new associations are made between atoms. Schematically, this reaction could be described asAB + CD → AD + CB, where A, B, C, and D are each unique atoms; and AB, AD, CD, and CB are each unique compounds.
^Whitten, Kenneth W.; Davis, Raymond E.; Peck, M. Larry (2000),General Chemistry (6th ed.), Fort Worth, TX: Saunders College Publishing/Harcourt College Publishers,ISBN978-0-03-072373-5
^Brown, Theodore L.; LeMay, H. Eugene; Bursten, Bruce E.; Murphy, Catherine J.; Woodward, Patrick (2013),Chemistry: The Central Science (3rd ed.), Frenchs Forest, NSW: Pearson/Prentice Hall, pp. 5–6,ISBN9781442559462,archived from the original on 2021-05-31, retrieved2020-12-08
^Hill, John W.; Petrucci, Ralph H.; McCreary, Terry W.; Perry, Scott S. (2005),General Chemistry (4th ed.), Upper Saddle River, NJ: Pearson/Prentice Hall, p. 6,ISBN978-0-13-140283-6,archived from the original on 2009-03-22
^Wilbraham, Antony; Matta, Michael; Staley, Dennis; Waterman, Edward (2002),Chemistry (1st ed.), Upper Saddle River, NJ: Pearson/Prentice Hall, p. 36,ISBN978-0-13-251210-7
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