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102-451: An acid is a molecule or ion capable of either donating a proton (i.e. hydrogen ion, H), known as a Brønsted–Lowry acid , or forming a covalent bond with an electron pair , known as a Lewis acid . The first category of acids are the proton donors, or Brønsted–Lowry acids . In the special case of aqueous solutions , proton donors form the hydronium ion H 3 O and are known as Arrhenius acids . Brønsted and Lowry generalized

204-440: A plane , e.g. graphene ; or three-dimensionally e.g. diamond , quartz , sodium chloride . The theme of repeated unit-cellular-structure also holds for most metals which are condensed phases with metallic bonding . Thus solid metals are not made of molecules. In glasses , which are solids that exist in a vitreous disordered state, the atoms are held together by chemical bonds with no presence of any definable molecule, nor any of

306-414: A spans many orders of magnitude, a more manageable constant, p K a is more frequently used, where p K a = −log 10 K a . Stronger acids have a smaller p K a than weaker acids. Experimentally determined p K a at 25 °C in aqueous solution are often quoted in textbooks and reference material. Arrhenius acids are named according to their anions . In the classical naming system,

408-450: A values are small, but K a1 > K a2 . A triprotic acid (H 3 A) can undergo one, two, or three dissociations and has three dissociation constants, where K a1 > K a2 > K a3 . An inorganic example of a triprotic acid is orthophosphoric acid (H 3 PO 4 ), usually just called phosphoric acid . All three protons can be successively lost to yield H 2 PO 4 , then HPO 4 , and finally PO 4 ,

510-492: A Lewis acid explicitly as such. Modern definitions are concerned with the fundamental chemical reactions common to all acids. Most acids encountered in everyday life are aqueous solutions , or can be dissolved in water, so the Arrhenius and Brønsted–Lowry definitions are the most relevant. The Brønsted–Lowry definition is the most widely used definition; unless otherwise specified, acid–base reactions are assumed to involve

612-399: A Lewis acid is an atomic or molecular species with a localized empty atomic or molecular orbital of low energy. This lowest-energy molecular orbital ( LUMO ) can accommodate a pair of electrons. A Lewis base is often a Brønsted–Lowry base as it can donate a pair of electrons to H ; the proton is a Lewis acid as it can accept a pair of electrons. The conjugate base of a Brønsted–Lowry acid

714-443: A Lewis acid, H, but at the same time, they also yield an equal amount of a Lewis base (acetate, citrate, or oxalate, respectively, for the acids mentioned). This article deals mostly with Brønsted acids rather than Lewis acids. Reactions of acids are often generalized in the form HA ⇌ H + A , where HA represents the acid and A is the conjugate base . This reaction is referred to as protolysis . The protonated form (HA) of an acid

816-636: A Lewis base and transfers a lone pair of electrons to form a bond with a hydrogen ion. The species that gains the electron pair is the Lewis acid; for example, the oxygen atom in H 3 O gains a pair of electrons when one of the H—O bonds is broken and the electrons shared in the bond become localized on oxygen. Depending on the context, a Lewis acid may also be described as an oxidizer or an electrophile . Organic Brønsted acids, such as acetic, citric, or oxalic acid, are not Lewis acids. They dissociate in water to produce

918-579: A benzene solvent and in the third gaseous HCl and NH 3 combine to form the solid. A third, only marginally related concept was proposed in 1923 by Gilbert N. Lewis , which includes reactions with acid–base characteristics that do not involve a proton transfer. A Lewis acid is a species that accepts a pair of electrons from another species; in other words, it is an electron pair acceptor. Brønsted acid–base reactions are proton transfer reactions while Lewis acid–base reactions are electron pair transfers. Many Lewis acids are not Brønsted–Lowry acids. Contrast how

1020-569: A certain number of these atoms united by attraction to form a single molecule . In coordination with these concepts, in 1833 the French chemist Marc Antoine Auguste Gaudin presented a clear account of Avogadro's hypothesis, regarding atomic weights, by making use of "volume diagrams", which clearly show both semi-correct molecular geometries, such as a linear water molecule, and correct molecular formulas, such as H 2 O: In 1917, an unknown American undergraduate chemical engineer named Linus Pauling

1122-433: A class of strong acids. A common example is toluenesulfonic acid (tosylic acid). Unlike sulfuric acid itself, sulfonic acids can be solids. In fact, polystyrene functionalized into polystyrene sulfonate is a solid strongly acidic plastic that is filterable. Superacids are acids stronger than 100% sulfuric acid. Examples of superacids are fluoroantimonic acid , magic acid and perchloric acid . The strongest known acid

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1224-510: A covalent bond with an electron pair. An example is boron trifluoride (BF 3 ), whose boron atom has a vacant orbital that can form a covalent bond by sharing a lone pair of electrons on an atom in a base, for example the nitrogen atom in ammonia (NH 3 ). Lewis considered this as a generalization of the Brønsted definition, so that an acid is a chemical species that accepts electron pairs either directly or by releasing protons (H) into

1326-399: A dimension of a few angstroms (Å) to several dozen Å, or around one billionth of a meter. Single molecules cannot usually be observed by light (as noted above), but small molecules and even the outlines of individual atoms may be traced in some circumstances by use of an atomic force microscope . Some of the largest molecules are macromolecules or supermolecules . The smallest molecule

1428-435: A graphical type of formula called a structural formula may be needed. Structural formulas may in turn be represented with a one-dimensional chemical name, but such chemical nomenclature requires many words and terms which are not part of chemical formulas. Molecules have fixed equilibrium geometries—bond lengths and angles— about which they continuously oscillate through vibrational and rotational motions. A pure substance

1530-657: A large application of Lewis bases is to modify the activity and selectivity of metal catalysts . Chiral Lewis bases, generally multidentate , confer chirality on a catalyst, enabling asymmetric catalysis , which is useful for the production of pharmaceuticals . The industrial synthesis of the anti-hypertension drug mibefradil uses a chiral Lewis base ( R -MeOBIPHEP), for example. Lewis acids and bases are commonly classified according to their hardness or softness. In this context hard implies small and nonpolarizable and soft indicates larger atoms that are more polarizable. For example, an amine will displace phosphine from

1632-586: A molecule is inherently an operational definition. Philosophically, therefore, a molecule is not a fundamental entity (in contrast, for instance, to an elementary particle ); rather, the concept of a molecule is the chemist's way of making a useful statement about the strengths of atomic-scale interactions in the world that we observe. Lewis base The terms nucleophile and electrophile are sometimes interchangeable with Lewis base and Lewis acid, respectively. These terms, especially their abstract noun forms nucleophilicity and electrophilicity , emphasize

1734-494: A more weakly bound Lewis base, often water. The proton (H )  is one of the strongest but is also one of the most complicated Lewis acids. It is convention to ignore the fact that a proton is heavily solvated (bound to solvent). With this simplification in mind, acid-base reactions can be viewed as the formation of adducts: A typical example of a Lewis acid in action is in the Friedel–Crafts alkylation reaction. The key step

1836-537: A pH of less than 7. While the Arrhenius concept is useful for describing many reactions, it is also quite limited in its scope. In 1923, chemists Johannes Nicolaus Brønsted and Thomas Martin Lowry independently recognized that acid–base reactions involve the transfer of a proton. A Brønsted–Lowry acid (or simply Brønsted acid) is a species that donates a proton to a Brønsted–Lowry base. Brønsted–Lowry acid–base theory has several advantages over Arrhenius theory. Consider

1938-590: A proton to ammonia (NH 3 ), but does not relate to the Arrhenius definition of an acid because the reaction does not produce hydronium. Nevertheless, CH 3 COOH is both an Arrhenius and a Brønsted–Lowry acid. Brønsted–Lowry theory can be used to describe reactions of molecular compounds in nonaqueous solution or the gas phase. Hydrogen chloride (HCl) and ammonia combine under several different conditions to form ammonium chloride , NH 4 Cl. In aqueous solution HCl behaves as hydrochloric acid and exists as hydronium and chloride ions. The following reactions illustrate

2040-513: A protonated acid to lose a proton, the pH of the system must rise above the p K a of the acid. The decreased concentration of H in that basic solution shifts the equilibrium towards the conjugate base form (the deprotonated form of the acid). In lower-pH (more acidic) solutions, there is a high enough H concentration in the solution to cause the acid to remain in its protonated form. Solutions of weak acids and salts of their conjugate bases form buffer solutions . Molecule A molecule

2142-401: A regular arrangement of molecules (as in a crystal). Microwave spectroscopy commonly measures changes in the rotation of molecules, and can be used to identify molecules in outer space. Infrared spectroscopy measures the vibration of molecules, including stretching, bending or twisting motions. It is commonly used to identify the kinds of bonds or functional groups in molecules. Changes in

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2244-461: A simple solution of an acid compound in water is determined by the dilution of the compound and the compound's K a . Lewis acids have been classified in the ECW model and it has been shown that there is no one order of acid strengths. The relative acceptor strength of Lewis acids toward a series of bases, versus other Lewis acids, can be illustrated by C-B plots . It has been shown that to define

2346-498: A solution with pH 7.0, which is only the case with similar acid and base strengths during a reaction. Neutralization with a base weaker than the acid results in a weakly acidic salt. An example is the weakly acidic ammonium chloride , which is produced from the strong acid hydrogen chloride and the weak base ammonia . Conversely, neutralizing a weak acid with a strong base gives a weakly basic salt (e.g., sodium fluoride from hydrogen fluoride and sodium hydroxide ). In order for

2448-443: A substance that increases the concentration of hydronium ions when added to water. Examples include molecular substances such as hydrogen chloride and acetic acid. An Arrhenius base , on the other hand, is a substance that increases the concentration of hydroxide (OH) ions when dissolved in water. This decreases the concentration of hydronium because the ions react to form H 2 O molecules: Due to this equilibrium, any increase in

2550-445: A type of Lewis acid. The IUPAC states that Lewis acids and Lewis bases react to form Lewis adducts, and defines electrophile as Lewis acids. Some of the most studied examples of such Lewis acids are the boron trihalides and organoboranes : In this adduct, all four fluoride centres (or more accurately, ligands ) are equivalent. Both BF 4 and BF 3 OMe 2 are Lewis base adducts of boron trifluoride. Many adducts violate

2652-454: A very large number of acidic protons. A diprotic acid (here symbolized by H 2 A) can undergo one or two dissociations depending on the pH. Each dissociation has its own dissociation constant, K a1 and K a2 . The first dissociation constant is typically greater than the second (i.e., K a1 > K a2 ). For example, sulfuric acid (H 2 SO 4 ) can donate one proton to form the bisulfate anion (HSO 4 ), for which K a1

2754-521: Is The W term represents a constant energy contribution for acid–base reaction such as the cleavage of a dimeric acid or base. The equation predicts reversal of acids and base strengths. The graphical presentations of the equation show that there is no single order of Lewis base strengths or Lewis acid strengths. and that single property scales are limited to a smaller range of acids or bases. The concept originated with Gilbert N. Lewis who studied chemical bonding . In 1923, Lewis wrote An acid substance

2856-416: Is helium hydride ion , with a proton affinity of 177.8kJ/mol. Superacids can permanently protonate water to give ionic, crystalline hydronium "salts". They can also quantitatively stabilize carbocations . While K a measures the strength of an acid compound, the strength of an aqueous acid solution is measured by pH, which is an indication of the concentration of hydronium in the solution. The pH of

2958-402: Is a chemical bond that involves the sharing of electron pairs between atoms. These electron pairs are termed shared pairs or bonding pairs , and the stable balance of attractive and repulsive forces between atoms, when they share electrons, is termed covalent bonding . Ionic bonding is a type of chemical bond that involves the electrostatic attraction between oppositely charged ions, and

3060-477: Is a group of two or more atoms that are held together by attractive forces known as chemical bonds ; depending on context, the term may or may not include ions that satisfy this criterion. In quantum physics , organic chemistry , and biochemistry , the distinction from ions is dropped and molecule is often used when referring to polyatomic ions . A molecule may be homonuclear , that is, it consists of atoms of one chemical element , e.g. two atoms in

3162-401: Is a substance that, when added to water, increases the concentration of H ions in the water. Chemists often write H( aq ) and refer to the hydrogen ion when describing acid–base reactions but the free hydrogen nucleus, a proton , does not exist alone in water, it exists as the hydronium ion (H 3 O) or other forms (H 5 O 2 , H 9 O 4 ). Thus, an Arrhenius acid can also be described as

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3264-455: Is a very simple type of chemical formula. It is the simplest integer ratio of the chemical elements that constitute it. For example, water is always composed of a 2:1 ratio of hydrogen to oxygen atoms, and ethanol (ethyl alcohol) is always composed of carbon, hydrogen, and oxygen in a 2:6:1 ratio. However, this does not determine the kind of molecule uniquely – dimethyl ether has the same ratios as ethanol, for instance. Molecules with

3366-516: Is also a Lewis base as loss of H from the acid leaves those electrons which were used for the A—H bond as a lone pair on the conjugate base. However, a Lewis base can be very difficult to protonate , yet still react with a Lewis acid. For example, carbon monoxide is a very weak Brønsted–Lowry base but it forms a strong adduct with BF 3 . In another comparison of Lewis and Brønsted–Lowry acidity by Brown and Kanner, 2,6-di- t -butylpyridine reacts to form

3468-423: Is also sometimes referred to as the free acid . Acid–base conjugate pairs differ by one proton, and can be interconverted by the addition or removal of a proton ( protonation and deprotonation , respectively). The acid can be the charged species and the conjugate base can be neutral in which case the generalized reaction scheme could be written as HA ⇌ H + A . In solution there exists an equilibrium between

3570-467: Is also used to represent hydrate coordination in various crystals, as in MgSO 4 ·7H 2 O for hydrated magnesium sulfate , irrespective of whether the water forms a dative bond with the metal. Although there have been attempts to use computational and experimental energetic criteria to distinguish dative bonding from non-dative covalent bonds, for the most part, the distinction merely makes note of

3672-435: Is composed of molecules with the same average geometrical structure. The chemical formula and the structure of a molecule are the two important factors that determine its properties, particularly its reactivity . Isomers share a chemical formula but normally have very different properties because of their different structures. Stereoisomers , a particular type of isomer, may have very similar physico-chemical properties and at

3774-483: Is derived from French molécule (1678), from Neo-Latin molecula , diminutive of Latin moles "mass, barrier". The word, which until the late 18th century was used only in Latin form, became popular after being used in works of philosophy by Descartes . The definition of the molecule has evolved as knowledge of the structure of molecules has increased. Earlier definitions were less precise, defining molecules as

3876-579: Is derived from the Latin acidus , meaning 'sour'. An aqueous solution of an acid has a pH less than 7 and is colloquially also referred to as "acid" (as in "dissolved in acid"), while the strict definition refers only to the solute . A lower pH means a higher acidity , and thus a higher concentration of positive hydrogen ions in the solution. Chemicals or substances having the property of an acid are said to be acidic . Common aqueous acids include hydrochloric acid (a solution of hydrogen chloride that

3978-486: Is essential for the understanding of the chemical bond. The simplest of molecules is the hydrogen molecule-ion , H 2 , and the simplest of all the chemical bonds is the one-electron bond . H 2 is composed of two positively charged protons and one negatively charged electron , which means that the Schrödinger equation for the system can be solved more easily due to the lack of electron–electron repulsion. With

4080-604: Is found in gastric acid in the stomach and activates digestive enzymes ), acetic acid (vinegar is a dilute aqueous solution of this liquid), sulfuric acid (used in car batteries ), and citric acid (found in citrus fruits). As these examples show, acids (in the colloquial sense) can be solutions or pure substances, and can be derived from acids (in the strict sense) that are solids, liquids, or gases. Strong acids and some concentrated weak acids are corrosive , but there are exceptions such as carboranes and boric acid . The second category of acids are Lewis acids , which form

4182-407: Is not very clear-cut. For example, in the formation of an ammonium ion from ammonia and hydrogen the ammonia molecule donates a pair of electrons to the proton ; the identity of the electrons is lost in the ammonium ion that is formed. Nevertheless, Lewis suggested that an electron-pair donor be classified as a base and an electron-pair acceptor be classified as acid. A more modern definition of

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4284-498: Is one which can employ an electron lone pair from another molecule in completing the stable group of one of its own atoms. The Brønsted–Lowry acid–base theory was published in the same year. The two theories are distinct but complementary. A Lewis base is also a Brønsted–Lowry base, but a Lewis acid does not need to be a Brønsted–Lowry acid. The classification into hard and soft acids and bases ( HSAB theory ) followed in 1963. The strength of Lewis acid-base interactions, as measured by

4386-474: Is simply added to the name of the ionic compound. Thus, for hydrogen chloride, as an acid solution, the IUPAC name is aqueous hydrogen chloride. The strength of an acid refers to its ability or tendency to lose a proton. A strong acid is one that completely dissociates in water; in other words, one mole of a strong acid HA dissolves in water yielding one mole of H and one mole of the conjugate base, A, and none of

4488-556: Is the diatomic hydrogen (H 2 ), with a bond length of 0.74 Å. Effective molecular radius is the size a molecule displays in solution. The table of permselectivity for different substances contains examples. The chemical formula for a molecule uses one line of chemical element symbols, numbers, and sometimes also other symbols, such as parentheses, dashes, brackets, and plus (+) and minus (−) signs. These are limited to one typographic line of symbols, which may include subscripts and superscripts. A compound's empirical formula

4590-441: Is the acceptance by AlCl 3 of a chloride ion lone-pair, forming AlCl − 4 and creating the strongly acidic, that is, electrophilic , carbonium ion. A Lewis base is an atomic or molecular species where the highest occupied molecular orbital (HOMO) is highly localized. Typical Lewis bases are conventional amines such as ammonia and alkyl amines. Other common Lewis bases include pyridine and its derivatives. Some of

4692-617: Is the primary interaction occurring in ionic compounds . The ions are atoms that have lost one or more electrons (termed cations ) and atoms that have gained one or more electrons (termed anions ). This transfer of electrons is termed electrovalence in contrast to covalence . In the simplest case, the cation is a metal atom and the anion is a nonmetal atom, but these ions can be of a more complicated nature, e.g. molecular ions like NH 4 or SO 4 . At normal temperatures and pressures, ionic bonding mostly creates solids (or occasionally liquids) without separate identifiable molecules, but

4794-422: Is very large; then it can donate a second proton to form the sulfate anion (SO 4 ), wherein the K a2 is intermediate strength. The large K a1 for the first dissociation makes sulfuric a strong acid. In a similar manner, the weak unstable carbonic acid (H 2 CO 3 ) can lose one proton to form bicarbonate anion (HCO 3 ) and lose a second to form carbonate anion (CO 3 ). Both K

4896-419: Is viewed as simply somewhere along a continuum between idealized covalent bonding and ionic bonding . Lewis acids are diverse and the term is used loosely. Simplest are those that react directly with the Lewis base, such as boron trihalides and the pentahalides of phosphorus, arsenic, and antimony. In the same vein, CH + 3 can be considered to be the Lewis acid in methylation reactions. However,

4998-534: The Avogadro constant using three different methods, all involving liquid phase systems. First, he used a gamboge soap-like emulsion, second by doing experimental work on Brownian motion , and third by confirming Einstein's theory of particle rotation in the liquid phase. In 1927, the physicists Fritz London and Walter Heitler applied the new quantum mechanics to the deal with the saturable, nondynamic forces of attraction and repulsion, i.e., exchange forces, of

5100-423: The citrate ion. Although the subsequent loss of each hydrogen ion is less favorable, all of the conjugate bases are present in solution. The fractional concentration, α (alpha), for each species can be calculated. For example, a generic diprotic acid will generate 3 species in solution: H 2 A, HA, and A. The fractional concentrations can be calculated as below when given either the pH (which can be converted to

5202-419: The kinetic aspect of reactivity, while the Lewis basicity and Lewis acidity emphasize the thermodynamic aspect of Lewis adduct formation. In many cases, the interaction between the Lewis base and Lewis acid in a complex is indicated by an arrow indicating the Lewis base donating electrons toward the Lewis acid using the notation of a dative bond — for example, Me 3 B ← NH 3 . Some sources indicate

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5304-410: The octet rule , such as the triiodide anion: The variability of the colors of iodine solutions reflects the variable abilities of the solvent to form adducts with the Lewis acid I 2 . Some Lewis acids bind with two Lewis bases, a famous example being the formation of hexafluorosilicate : Most compounds considered to be Lewis acids require an activation step prior to formation of the adduct with

5406-684: The oxygen molecule (O 2 ); or it may be heteronuclear , a chemical compound composed of more than one element, e.g. water (two hydrogen atoms and one oxygen atom; H 2 O). In the kinetic theory of gases , the term molecule is often used for any gaseous particle regardless of its composition. This relaxes the requirement that a molecule contains two or more atoms, since the noble gases are individual atoms. Atoms and complexes connected by non-covalent interactions , such as hydrogen bonds or ionic bonds , are typically not considered single molecules. Concepts similar to molecules have been discussed since ancient times, but modern investigation into

5508-476: The Arrhenius theory to include non-aqueous solvents . A Brønsted or Arrhenius acid usually contains a hydrogen atom bonded to a chemical structure that is still energetically favorable after loss of H. Aqueous Arrhenius acids have characteristic properties that provide a practical description of an acid. Acids form aqueous solutions with a sour taste, can turn blue litmus red, and react with bases and certain metals (like calcium ) to form salts . The word acid

5610-476: The Chemical Bond" in which he used quantum mechanics to calculate properties and structures of molecules, such as angles between bonds and rotation about bonds. On these concepts, Pauling developed hybridization theory to account for bonds in molecules such as CH 4 , in which four sp³ hybridised orbitals are overlapped by hydrogen 's 1s orbital, yielding four sigma (σ) bonds . The four bonds are of

5712-408: The Lewis base with a pair of dots (the explicit electrons being donated), which allows consistent representation of the transition from the base itself to the complex with the acid: A center dot may also be used to represent a Lewis adduct, such as Me 3 B·NH 3 . Another example is boron trifluoride diethyl etherate , BF 3 ·Et 2 O . In a slightly different usage, the center dot

5814-523: The Lewis base. Complex compounds such as Et 3 Al 2 Cl 3 and AlCl 3 are treated as trigonal planar Lewis acids but exist as aggregates and polymers that must be degraded by the Lewis base. A simpler case is the formation of adducts of borane. Monomeric BH 3 does not exist appreciably, so the adducts of borane are generated by degradation of diborane: In this case, an intermediate B 2 H − 7 can be isolated. Many metal complexes serve as Lewis acids, but usually only after dissociating

5916-405: The [H]) or the concentrations of the acid with all its conjugate bases: A plot of these fractional concentrations against pH, for given K 1 and K 2 , is known as a Bjerrum plot . A pattern is observed in the above equations and can be expanded to the general n -protic acid that has been deprotonated i -times: where K 0 = 1 and the other K-terms are the dissociation constants for

6018-424: The acid and its conjugate base. The equilibrium constant K is an expression of the equilibrium concentrations of the molecules or the ions in solution. Brackets indicate concentration, such that [H 2 O] means the concentration of H 2 O . The acid dissociation constant K a is generally used in the context of acid–base reactions. The numerical value of K a is equal to the product (multiplication) of

6120-435: The acid. Neutralization is the reaction between an acid and a base, producing a salt and neutralized base; for example, hydrochloric acid and sodium hydroxide form sodium chloride and water: Neutralization is the basis of titration , where a pH indicator shows equivalence point when the equivalent number of moles of a base have been added to an acid. It is often wrongly assumed that neutralization should result in

6222-501: The adduct with the acid BF 3 . In the same way, bases could be classified. For example, bases donating a lone pair from an oxygen atom are harder than bases donating through a nitrogen atom. Although the classification was never quantified it proved to be very useful in predicting the strength of adduct formation, using the key concepts that hard acid—hard base and soft acid—soft base interactions are stronger than hard acid—soft base or soft acid—hard base interactions. Later investigation of

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6324-441: The arrangements of electrons yield absorption or emission lines in ultraviolet, visible or near infrared light, and result in colour. Nuclear resonance spectroscopy measures the environment of particular nuclei in the molecule, and can be used to characterise the numbers of atoms in different positions in a molecule. The study of molecules by molecular physics and theoretical chemistry is largely based on quantum mechanics and

6426-437: The concentration of hydronium is accompanied by a decrease in the concentration of hydroxide. Thus, an Arrhenius acid could also be said to be one that decreases hydroxide concentration, while an Arrhenius base increases it. In an acidic solution, the concentration of hydronium ions is greater than 10 moles per liter. Since pH is defined as the negative logarithm of the concentration of hydronium ions, acidic solutions thus have

6528-422: The concentrations of the products divided by the concentration of the reactants, where the reactant is the acid (HA) and the products are the conjugate base and H. The stronger of two acids will have a higher K a than the weaker acid; the ratio of hydrogen ions to acid will be higher for the stronger acid as the stronger acid has a greater tendency to lose its proton. Because the range of possible values for K

6630-484: The development of fast digital computers, approximate solutions for more complicated molecules became possible and are one of the main aspects of computational chemistry . When trying to define rigorously whether an arrangement of atoms is sufficiently stable to be considered a molecule, IUPAC suggests that it "must correspond to a depression on the potential energy surface that is deep enough to confine at least one vibrational state". This definition does not depend on

6732-413: The fluoride nucleus than they are in the lone fluoride ion. BF 3 is a Lewis acid because it accepts the electron pair from fluoride. This reaction cannot be described in terms of Brønsted theory because there is no proton transfer. The second reaction can be described using either theory. A proton is transferred from an unspecified Brønsted acid to ammonia, a Brønsted base; alternatively, ammonia acts as

6834-448: The following reactions are described in terms of acid–base chemistry: In the first reaction a fluoride ion , F, gives up an electron pair to boron trifluoride to form the product tetrafluoroborate . Fluoride "loses" a pair of valence electrons because the electrons shared in the B—F bond are located in the region of space between the two atomic nuclei and are therefore more distant from

6936-426: The following reactions of acetic acid (CH 3 COOH), the organic acid that gives vinegar its characteristic taste: Both theories easily describe the first reaction: CH 3 COOH acts as an Arrhenius acid because it acts as a source of H 3 O when dissolved in water, and it acts as a Brønsted acid by donating a proton to water. In the second example CH 3 COOH undergoes the same transformation, in this case donating

7038-464: The heavenly bodies. The viewpoint of Leucippus and Empedocles, along with the aether, was accepted by Aristotle and passed to medieval and renaissance Europe. In a more concrete manner, however, the concept of aggregates or units of bonded atoms, i.e. "molecules", traces its origins to Robert Boyle 's 1661 hypothesis, in his famous treatise The Sceptical Chymist , that matter is composed of clusters of particles and that chemical change results from

7140-537: The hydrogen molecule. Their valence bond treatment of this problem, in their joint paper, was a landmark in that it brought chemistry under quantum mechanics. Their work was an influence on Pauling, who had just received his doctorate and visited Heitler and London in Zürich on a Guggenheim Fellowship . Subsequently, in 1931, building on the work of Heitler and London and on theories found in Lewis' famous article, Pauling published his ground-breaking article "The Nature of

7242-454: The ionic suffix is dropped and replaced with a new suffix, according to the table following. The prefix "hydro-" is used when the acid is made up of just hydrogen and one other element. For example, HCl has chloride as its anion, so the hydro- prefix is used, and the -ide suffix makes the name take the form hydrochloric acid . Classical naming system: In the IUPAC naming system, "aqueous"

7344-735: The laws governing their structure and properties. In practice, however, this distinction is vague. In molecular sciences, a molecule consists of a stable system ( bound state ) composed of two or more atoms. Polyatomic ions may sometimes be usefully thought of as electrically charged molecules. The term unstable molecule is used for very reactive species, i.e., short-lived assemblies ( resonances ) of electrons and nuclei , such as radicals , molecular ions , Rydberg molecules , transition states , van der Waals complexes , or systems of colliding atoms as in Bose–Einstein condensate . Molecules as components of matter are common. They also make up most of

7446-467: The limitations of Arrhenius's definition: As with the acetic acid reactions, both definitions work for the first example, where water is the solvent and hydronium ion is formed by the HCl solute. The next two reactions do not involve the formation of ions but are still proton-transfer reactions. In the second reaction hydrogen chloride and ammonia (dissolved in benzene ) react to form solid ammonium chloride in

7548-504: The main classes of Lewis bases are The most common Lewis bases are anions. The strength of Lewis basicity correlates with the pK a of the parent acid: acids with high pK a 's give good Lewis bases. As usual, a weaker acid has a stronger conjugate base . The strength of Lewis bases have been evaluated for various Lewis acids, such as I 2 , SbCl 5 , and BF 3 . Nearly all electron pair donors that form compounds by binding transition elements can be viewed ligands . Thus,

7650-404: The mass of a neutral carbon-12 ( C isotope ) atom. For network solids , the term formula unit is used in stoichiometric calculations. For molecules with a complicated 3-dimensional structure, especially involving atoms bonded to four different substituents, a simple molecular formula or even semi-structural chemical formula may not be enough to completely specify the molecule. In this case,

7752-450: The methyl cation never occurs as a free species in the condensed phase, and methylation reactions by reagents like CH 3 I take place through the simultaneous formation of a bond from the nucleophile to the carbon and cleavage of the bond between carbon and iodine (S N 2 reaction). Textbooks disagree on this point: some asserting that alkyl halides are electrophiles but not Lewis acids, while others describe alkyl halides (e.g. CH 3 Br) as

7854-452: The minerals that make up the substance of the Earth, sand, clay, pebbles, rocks, boulders, bedrock , the molten interior , and the core of the Earth . All of these contain many chemical bonds, but are not made of identifiable molecules. No typical molecule can be defined for salts nor for covalent crystals , although these are often composed of repeating unit cells that extend either in

7956-502: The more easily it loses a proton, H. Two key factors that contribute to the ease of deprotonation are the polarity of the H—A bond and the size of atom A, which determines the strength of the H—A bond. Acid strengths are also often discussed in terms of the stability of the conjugate base. Stronger acids have a larger acid dissociation constant , K a and a lower p K a than weaker acids. Sulfonic acids , which are organic oxyacids, are

8058-501: The nature of molecules and their bonds began in the 17th century. Refined over time by scientists such as Robert Boyle , Amedeo Avogadro , Jean Perrin , and Linus Pauling , the study of molecules is today known as molecular physics or molecular chemistry. According to Merriam-Webster and the Online Etymology Dictionary , the word "molecule" derives from the Latin " moles " or small unit of mass. The word

8160-440: The nature of the interaction between the atoms, but only on the strength of the interaction. In fact, it includes weakly bound species that would not traditionally be considered molecules, such as the helium dimer , He 2 , which has one vibrational bound state and is so loosely bound that it is only likely to be observed at very low temperatures. Whether or not an arrangement of atoms is sufficiently stable to be considered

8262-517: The oceans and atmosphere. Most organic substances are molecules. The substances of life are molecules, e.g. proteins, the amino acids of which they are composed, the nucleic acids (DNA and RNA), sugars, carbohydrates, fats, and vitamins. The nutrient minerals are generally ionic compounds, thus they are not molecules, e.g. iron sulfate. However, the majority of familiar solid substances on Earth are made partly or completely of crystals or ionic compounds, which are not made of molecules. These include all of

8364-600: The order of Lewis acid strength at least two properties must be considered. For Pearson's qualitative HSAB theory the two properties are hardness and strength while for Drago's quantitative ECW model the two properties are electrostatic and covalent. Monoprotic acids, also known as monobasic acids, are those acids that are able to donate one proton per molecule during the process of dissociation (sometimes called ionization) as shown below (symbolized by HA): Common examples of monoprotic acids in mineral acids include hydrochloric acid (HCl) and nitric acid (HNO 3 ). On

8466-423: The orthophosphate ion, usually just called phosphate . Even though the positions of the three protons on the original phosphoric acid molecule are equivalent, the successive K a values differ since it is energetically less favorable to lose a proton if the conjugate base is more negatively charged. An organic example of a triprotic acid is citric acid , which can successively lose three protons to finally form

8568-740: The other hand, for organic acids the term mainly indicates the presence of one carboxylic acid group and sometimes these acids are known as monocarboxylic acid. Examples in organic acids include formic acid (HCOOH), acetic acid (CH 3 COOH) and benzoic acid (C 6 H 5 COOH). Polyprotic acids, also known as polybasic acids, are able to donate more than one proton per acid molecule, in contrast to monoprotic acids that only donate one proton per molecule. Specific types of polyprotic acids have more specific names, such as diprotic (or dibasic) acid (two potential protons to donate), and triprotic (or tribasic) acid (three potential protons to donate). Some macromolecules such as proteins and nucleic acids can have

8670-423: The protonated acid HA. In contrast, a weak acid only partially dissociates and at equilibrium both the acid and the conjugate base are in solution. Examples of strong acids are hydrochloric acid (HCl), hydroiodic acid (HI), hydrobromic acid (HBr), perchloric acid (HClO 4 ), nitric acid (HNO 3 ) and sulfuric acid (H 2 SO 4 ). In water each of these essentially ionizes 100%. The stronger an acid is,

8772-411: The rearrangement of the clusters. Boyle argued that matter's basic elements consisted of various sorts and sizes of particles, called "corpuscles", which were capable of arranging themselves into groups. In 1789, William Higgins published views on what he called combinations of "ultimate" particles, which foreshadowed the concept of valency bonds . If, for example, according to Higgins, the force between

8874-546: The regularity of repeating unit-cellular-structure that characterizes salts, covalent crystals, and metals. Molecules are generally held together by covalent bonding . Several non-metallic elements exist only as molecules in the environment either in compounds or as homonuclear molecules, not as free atoms: for example, hydrogen. While some people say a metallic crystal can be considered a single giant molecule held together by metallic bonding , others point out that metals behave very differently than molecules. A covalent bond

8976-405: The same atoms in different arrangements are called isomers . Also carbohydrates, for example, have the same ratio (carbon:hydrogen:oxygen= 1:2:1) (and thus the same empirical formula) but different total numbers of atoms in the molecule. The molecular formula reflects the exact number of atoms that compose the molecule and so characterizes different molecules. However different isomers can have

9078-404: The same atomic composition while being different molecules. The empirical formula is often the same as the molecular formula but not always. For example, the molecule acetylene has molecular formula C 2 H 2 , but the simplest integer ratio of elements is CH. The molecular mass can be calculated from the chemical formula and is expressed in conventional atomic mass units equal to 1/12 of

9180-405: The same length and strength, which yields a molecular structure as shown below: The science of molecules is called molecular chemistry or molecular physics , depending on whether the focus is on chemistry or physics. Molecular chemistry deals with the laws governing the interaction between molecules that results in the formation and breakage of chemical bonds, while molecular physics deals with

9282-572: The same time different biochemical activities. Molecular spectroscopy deals with the response ( spectrum ) of molecules interacting with probing signals of known energy (or frequency , according to the Planck relation ). Molecules have quantized energy levels that can be analyzed by detecting the molecule's energy exchange through absorbance or emission . Spectroscopy does not generally refer to diffraction studies where particles such as neutrons , electrons, or high energy X-rays interact with

9384-517: The smallest particles of pure chemical substances that still retain their composition and chemical properties. This definition often breaks down since many substances in ordinary experience, such as rocks , salts , and metals , are composed of large crystalline networks of chemically bonded atoms or ions , but are not made of discrete molecules. The modern concept of molecules can be traced back towards pre-scientific and Greek philosophers such as Leucippus and Democritus who argued that all

9486-405: The solution, which then accept electron pairs. Hydrogen chloride, acetic acid, and most other Brønsted–Lowry acids cannot form a covalent bond with an electron pair, however, and are therefore not Lewis acids. Conversely, many Lewis acids are not Arrhenius or Brønsted–Lowry acids. In modern terminology, an acid is implicitly a Brønsted acid and not a Lewis acid, since chemists almost always refer to

9588-417: The source of the electron pair, and dative bonds, once formed, behave simply as other covalent bonds do, though they typically have considerable polar character. Moreover, in some cases (e.g., sulfoxides and amine oxides as R 2 S → O and R 3 N → O ), the use of the dative bond arrow is just a notational convenience for avoiding the drawing of formal charges. In general, however, the donor–acceptor bond

9690-493: The standard enthalpy of formation of an adduct can be predicted by the Drago–Wayland two-parameter equation. Lewis had suggested in 1916 that two atoms are held together in a chemical bond by sharing a pair of electrons. When each atom contributed one electron to the bond, it was called a covalent bond . When both electrons come from one of the atoms, it was called a dative covalent bond or coordinate bond . The distinction

9792-407: The strength of Lewis acid base interactions, −ΔH. The model assigned E and C parameters to many Lewis acids and bases. Each acid is characterized by an E A and a C A . Each base is likewise characterized by its own E B and C B . The E and C parameters refer, respectively, to the electrostatic and covalent contributions to the strength of the bonds that the acid and base will form. The equation

9894-540: The thermodynamics of the interaction suggested that hard—hard interactions are enthalpy favored, whereas soft—soft are entropy favored. Many methods have been devised to evaluate and predict Lewis acidity. Many are based on spectroscopic signatures such as shifts NMR signals or IR bands e.g. the Gutmann-Beckett method and the Childs method. The ECW model is a quantitative model that describes and predicts

9996-437: The transfer of a proton (H) from an acid to a base. Hydronium ions are acids according to all three definitions. Although alcohols and amines can be Brønsted–Lowry acids, they can also function as Lewis bases due to the lone pairs of electrons on their oxygen and nitrogen atoms. In 1884, Svante Arrhenius attributed the properties of acidity to hydrogen ions (H), later described as protons or hydrons . An Arrhenius acid

10098-668: The ultimate particle of oxygen and the ultimate particle of nitrogen were 6, then the strength of the force would be divided accordingly, and similarly for the other combinations of ultimate particles. Amedeo Avogadro created the word "molecule". His 1811 paper "Essay on Determining the Relative Masses of the Elementary Molecules of Bodies", he essentially states, i.e. according to Partington 's A Short History of Chemistry , that: The smallest particles of gases are not necessarily simple atoms, but are made up of

10200-409: The universe is composed of atoms and voids . Circa 450 BC Empedocles imagined fundamental elements ( fire ( [REDACTED] ), earth ( [REDACTED] ), air ( [REDACTED] ), and water ( [REDACTED] )) and "forces" of attraction and repulsion allowing the elements to interact. A fifth element, the incorruptible quintessence aether , was considered to be the fundamental building block of

10302-432: The vaporization/sublimation of such materials does produce separate molecules where electrons are still transferred fully enough for the bonds to be considered ionic rather than covalent. Most molecules are far too small to be seen with the naked eye, although molecules of many polymers can reach macroscopic sizes, including biopolymers such as DNA . Molecules commonly used as building blocks for organic synthesis have

10404-526: Was learning the Dalton hook-and-eye bonding method , which was the mainstream description of bonds between atoms at the time. Pauling, however, was not satisfied with this method and looked to the newly emerging field of quantum physics for a new method. In 1926, French physicist Jean Perrin received the Nobel Prize in physics for proving, conclusively, the existence of molecules. He did this by calculating

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