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122-411: In chemistry , valence bond (VB) theory is one of the two basic theories, along with molecular orbital (MO) theory , that were developed to use the methods of quantum mechanics to explain chemical bonding . It focuses on how the atomic orbitals of the dissociated atoms combine to give individual chemical bonds when a molecule is formed. In contrast, molecular orbital theory has orbitals that cover

244-419: A 3c-4e bond in xenon difluoride . The allyl cation has two contributing structures with a positive charge on the terminal carbon atoms. In the hybrid structure their charge is + 1 ⁄ 2 . The full positive charge can also be depicted as delocalized among three carbon atoms. The diborane molecule is described by contributing structures, each with electron-deficiency on different atoms. This reduces

366-440: A chemical equation . While in a non-nuclear chemical reaction the number and kind of atoms on both sides of the equation are equal, for a nuclear reaction this holds true only for the nuclear particles viz. protons and neutrons. The sequence of steps in which the reorganization of chemical bonds may be taking place in the course of a chemical reaction is called its mechanism . A chemical reaction can be envisioned to take place in

488-402: A chemical equilibrium . Alternatively to the use of contributing structures in diagrams, a hybrid structure can be used. In a hybrid structure, pi bonds that are involved in resonance are usually pictured as curves or dashed lines, indicating that these are partial rather than normal complete pi bonds. In benzene and other aromatic rings, the delocalized pi-electrons are sometimes pictured as

610-572: A contributing structure and the resonance hybrid becomes the hybrid structure . The double headed arrows would be replaced by commas to illustrate a set of structures, as arrows of any type may suggest that a chemical change is taking place. In diagrams, contributing structures are typically separated by double-headed arrows (↔). The arrow should not be confused with the right and left pointing equilibrium arrow (⇌). All structures together may be enclosed in large square brackets, to indicate they picture one single molecule or ion, not different species in

732-479: A covalent bond . He then called up his associate Fritz London and they worked out the details of the theory over the course of the night. Later, Linus Pauling used the pair bonding ideas of Lewis together with Heitler–London theory to develop two other key concepts in VB theory: resonance (1928) and orbital hybridization (1930). According to Charles Coulson , author of the noted 1952 book Valence , this period marks

854-417: A C–C single bond is 154 pm ; that of a C=C double bond is 133 pm. In localized cyclohexatriene, the carbon–carbon bonds should be alternating 154 and 133 pm. Instead, all carbon–carbon bonds in benzene are found to be about 139 pm, a bond length intermediate between single and double bond. This mixed single and double bond (or triple bond) character is typical for all molecules in which bonds have

976-459: A bond between two s -orbital electrons is a sigma bond, because two spheres are always coaxial. In terms of bond order, single bonds have one sigma bond, double bonds consist of one sigma bond and one pi bond, and triple bonds contain one sigma bond and two pi bonds. However, the atomic orbitals for bonding may be hybrids. Often, the bonding atomic orbitals have a character of several possible types of orbitals. The methods to get an atomic orbital with

1098-467: A chemical reaction is said to have occurred. A chemical reaction is therefore a concept related to the "reaction" of a substance when it comes in close contact with another, whether as a mixture or a solution ; exposure to some form of energy, or both. It results in some energy exchange between the constituents of the reaction as well as with the system environment, which may be designed vessels—often laboratory glassware . Chemical reactions can result in

1220-470: A chemical transformation is the rearrangement of electrons in the chemical bonds between atoms. It can be symbolically depicted through a chemical equation , which usually involves atoms as subjects. The number of atoms on the left and the right in the equation for a chemical transformation is equal. (When the number of atoms on either side is unequal, the transformation is referred to as a nuclear reaction or radioactive decay .) The type of chemical reactions

1342-529: A clearer and more reliable framework for predicting magnetic and ionization properties. In particular, MO theory can effectively account for paramagnetism arising from unpaired electrons, whereas VBT struggles. Valence bond theory views aromatic properties of molecules as due to spin coupling of the π orbitals . This is essentially still the old idea of resonance between Friedrich August Kekulé von Stradonitz and James Dewar structures. In contrast, molecular orbital theory views aromaticity as delocalization of

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1464-412: A dense core called the atomic nucleus surrounded by a space occupied by an electron cloud . The nucleus is made up of positively charged protons and uncharged neutrons (together called nucleons ), while the electron cloud consists of negatively charged electrons which orbit the nucleus. In a neutral atom, the negatively charged electrons balance out the positive charge of the protons. The nucleus

1586-428: A different bond order in different contributing structures. Bond lengths can be compared using bond orders. For example, in cyclohexane the bond order is 1 while that in benzene is 1 + (3 ÷ 6) = 1 + 1 ⁄ 2 . Consequently, benzene has more double bond character and hence has a shorter bond length than cyclohexane. Resonance (or delocalization) energy is the amount of energy needed to convert

1708-535: A directed beam in a vacuum in a mass spectrometer . Charged polyatomic collections residing in solids (for example, common sulfate or nitrate ions) are generally not considered "molecules" in chemistry. Some molecules contain one or more unpaired electrons, creating radicals . Most radicals are comparatively reactive, but some, such as nitric oxide (NO) can be stable. The "inert" or noble gas elements ( helium , neon , argon , krypton , xenon and radon ) are composed of lone atoms as their smallest discrete unit, but

1830-514: A formalism that does not necessarily reflect the true electronic structure, such depictions are preferred by the IUPAC over structures featuring partial bonds, charge separation, or dative bonds . Equivalent contributors contribute equally to the actual structure, while the importance of nonequivalent contributors is determined by the extent to which they conform to the properties listed above. A larger number of significant contributing structures and

1952-801: A fundamental level. For example, chemistry explains aspects of plant growth ( botany ), the formation of igneous rocks ( geology ), how atmospheric ozone is formed and how environmental pollutants are degraded ( ecology ), the properties of the soil on the Moon ( cosmochemistry ), how medications work ( pharmacology ), and how to collect DNA evidence at a crime scene ( forensics ). Chemistry has existed under various names since ancient times. It has evolved, and now chemistry encompasses various areas of specialisation, or subdisciplines, that continue to increase in number and interrelate to create further interdisciplinary fields of study. The applications of various fields of chemistry are used frequently for economic purposes in

2074-417: A large number of basis functions , either centered each on one atom to give a classical valence bond picture, or centered on all atoms in the molecule. The resulting energies are more competitive with energies from calculations where electron correlation is introduced based on a Hartree–Fock reference wavefunction. The most recent text is by Shaik and Hiberty. An important aspect of the valence bond theory

2196-433: A mixture of atoms and ions. For example, the molecular orbital function for dihydrogen is an equal mixture of the covalent and ionic valence bond structures and so predicts incorrectly that the molecule would dissociate into an equal mixture of hydrogen atoms and hydrogen positive and negative ions. Modern valence bond theory replaces the overlapping atomic orbitals by overlapping valence bond orbitals that are expanded over

2318-409: A more voluminous space available for delocalized electrons lead to stabilization (lowering of the energy) of the molecule. In benzene the two cyclohexatriene Kekulé structures, first proposed by Kekulé , are taken together as contributing structures to represent the total structure. In the hybrid structure on the right, the dashed hexagon replaces three double bonds, and represents six electrons in

2440-411: A number of steps, each of which may have a different speed. Many reaction intermediates with variable stability can thus be envisaged during the course of a reaction. Reaction mechanisms are proposed to explain the kinetics and the relative product mix of a reaction. Many physical chemists specialize in exploring and proposing the mechanisms of various chemical reactions. Several empirical rules, like

2562-424: A particular substance per volume of solution , and is commonly reported in mol/ dm . In addition to the specific chemical properties that distinguish different chemical classifications, chemicals can exist in several phases. For the most part, the chemical classifications are independent of these bulk phase classifications; however, some more exotic phases are incompatible with certain chemical properties. A phase

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2684-424: A positive hydrogen ion to another substance in a chemical reaction; by extension, a base is the substance which receives that hydrogen ion. Resonance (chemistry) Under the framework of valence bond theory , resonance is an extension of the idea that the bonding in a chemical species can be described by a Lewis structure. For many chemical species, a single Lewis structure, consisting of atoms obeying

2806-405: A pure chemical substance that has its unique set of chemical properties, that is, its potential to undergo a certain set of chemical reactions with other substances. However, this definition only works well for substances that are composed of molecules, which is not true of many substances (see below). Molecules are typically a set of atoms bound together by covalent bonds , such that the structure

2928-444: A set of three molecular orbitals of π symmetry, with a nodal plane in the plane of the molecule. In furan a lone pair of the oxygen atom interacts with the π orbitals of the carbon atoms. The curved arrows depict the permutation of delocalized π electrons , which results in different contributors. The ozone molecule is represented by two contributing structures. In reality the two terminal oxygen atoms are equivalent and

3050-437: A single Lewis structure cannot be written, several valence bond structures are used. Each of these VB structures represents a specific Lewis structure. This combination of valence bond structures is the main point of resonance theory. Valence bond theory considers that the overlapping atomic orbitals of the participating atoms form a chemical bond . Because of the overlapping, it is most probable that electrons should be in

3172-484: A solid circle. The concept first appeared in 1899 in Johannes Thiele 's "Partial Valence Hypothesis" to explain the unusual stability of benzene which would not be expected from August Kekulé 's structure proposed in 1865 with alternating single and double bonds. Benzene undergoes substitution reactions, rather than addition reactions as typical for alkenes . He proposed that the carbon-carbon bond in benzene

3294-609: A substance may undergo and the energy changes that may accompany it are constrained by certain basic rules, known as chemical laws . Energy and entropy considerations are invariably important in almost all chemical studies. Chemical substances are classified in terms of their structure , phase, as well as their chemical compositions . They can be analyzed using the tools of chemical analysis , e.g. spectroscopy and chromatography . Scientists engaged in chemical research are known as chemists . Most chemists specialize in one or more sub-disciplines. Several concepts are essential for

3416-405: A system, any species represented by a resonance hybrid is more stable than any of the (hypothetical) contributing structures. Electron delocalization stabilizes a molecule because the electrons are more evenly spread out over the molecule, decreasing electron-electron repulsion. The difference in potential energy between the actual species and the (computed) energy of the contributing structure with

3538-534: Is a bit higher: 151 kJ or 36 kcal/mol. This measured resonance energy is also the difference between the hydrogenation energy of three 'non-resonance' double bonds and the measured hydrogenation energy: Regardless of their exact values, resonance energies of various related compounds provide insights into their bonding. The resonance energies for pyrrole , thiophene , and furan are, respectively, 88, 121, and 67 kJ/mol (21, 29, and 16 kcal/mol). Thus, these heterocycles are far less aromatic than benzene, as

3660-414: Is a pure substance which is composed of a single type of atom, characterized by its particular number of protons in the nuclei of its atoms, known as the atomic number and represented by the symbol Z . The mass number is the sum of the number of protons and neutrons in a nucleus. Although all the nuclei of all atoms belonging to one element will have the same atomic number, they may not necessarily have

3782-409: Is a set of states of a chemical system that have similar bulk structural properties, over a range of conditions, such as pressure or temperature . Physical properties, such as density and refractive index tend to fall within values characteristic of the phase. The phase of matter is defined by the phase transition , which is when energy put into or taken out of the system goes into rearranging

Valence bond theory - Misplaced Pages Continue

3904-586: Is a very useful means for predicting the feasibility of a reaction and determining the state of equilibrium of a chemical reaction, in chemical thermodynamics . A reaction is feasible only if the total change in the Gibbs free energy is negative, Δ G ≤ 0 {\displaystyle \Delta G\leq 0\,} ; if it is equal to zero the chemical reaction is said to be at equilibrium . There exist only limited possible states of energy for electrons, atoms and molecules. These are determined by

4026-422: Is also possible to define analogs in two-dimensional systems, which has received attention for its relevance to systems in biology . Atoms sticking together in molecules or crystals are said to be bonded with one another. A chemical bond may be visualized as the multipole balance between the positive charges in the nuclei and the negative charges oscillating about them. More than simple attraction and repulsion,

4148-439: Is also used to identify the composition of remote objects – like stars and distant galaxies – by analyzing their radiation spectra. The term chemical energy is often used to indicate the potential of a chemical substance to undergo a transformation through a chemical reaction or to transform other chemical substances. When a chemical substance is transformed as a result of its interaction with another substance or with energy,

4270-478: Is available on the HuLiS Web site. In the case of ions it is common to speak about delocalized charge (charge delocalization). An example of delocalized charge in ions can be found in the carboxylate group, wherein the negative charge is centered equally on the two oxygen atoms. Charge delocalization in anions is an important factor determining their reactivity (generally: the higher the extent of delocalization

4392-464: Is called a mixture. Examples of mixtures are air and alloys . The mole is a unit of measurement that denotes an amount of substance (also called chemical amount). One mole is defined to contain exactly 6.022 140 76 × 10 particles ( atoms , molecules , ions , or electrons ), where the number of particles per mole is known as the Avogadro constant . Molar concentration is the amount of

4514-523: Is composed of gaseous matter that has been completely ionized, usually through high temperature. A substance can often be classified as an acid or a base . There are several different theories which explain acid–base behavior. The simplest is Arrhenius theory , which states that acid is a substance that produces hydronium ions when it is dissolved in water, and a base is one that produces hydroxide ions when dissolved in water. According to Brønsted–Lowry acid–base theory , acids are substances that donate

4636-472: Is dense; the mass of a nucleon is approximately 1,836 times that of an electron, yet the radius of an atom is about 10,000 times that of its nucleus. The atom is also the smallest entity that can be envisaged to retain the chemical properties of the element, such as electronegativity , ionization potential , preferred oxidation state (s), coordination number , and preferred types of bonds to form (e.g., metallic , ionic , covalent ). A chemical element

4758-464: Is electrically neutral and all valence electrons are paired with other electrons either in bonds or in lone pairs . Thus, molecules exist as electrically neutral units, unlike ions. When this rule is broken, giving the "molecule" a charge, the result is sometimes named a molecular ion or a polyatomic ion. However, the discrete and separate nature of the molecular concept usually requires that molecular ions be present only in well-separated form, such as

4880-423: Is incorrect to regard resonance hybrids as rapidly interconverting isomers, even though the term "resonance" might evoke such an image. (As described below , the term "resonance" originated as a classical physics analogy for a quantum mechanical phenomenon, so it should not be construed too literally.) Symbolically, the double headed arrow A ⟷ B {\displaystyle {\ce {A<->B}}}

5002-415: Is intermediate of a single and double bond. The resonance proposal also helped explain the number of isomers of benzene derivatives. For example, Kekulé's structure would predict four dibromobenzene isomers, including two ortho isomers with the brominated carbon atoms joined by either a single or a double bond. In reality there are only three dibromobenzene isomers and only one is ortho, in agreement with

Valence bond theory - Misplaced Pages Continue

5124-434: Is manifested in the lability of these rings. Resonance has a deeper significance in the mathematical formalism of valence bond theory (VB). Quantum mechanics requires that the wavefunction of a molecule obey its observed symmetry. If a single contributing structure does not achieve this, resonance is invoked. For example, in benzene, valence bond theory begins with the two Kekulé structures which do not individually possess

5246-537: Is more easily transferred between substances relative to light or other forms of electronic energy. For example, ultraviolet electromagnetic radiation is not transferred with as much efficacy from one substance to another as thermal or electrical energy. The existence of characteristic energy levels for different chemical substances is useful for their identification by the analysis of spectral lines . Different kinds of spectra are often used in chemical spectroscopy , e.g. IR , microwave , NMR , ESR , etc. Spectroscopy

5368-842: Is no mathematical formula either in chemistry or quantum mechanics for the arrangement of electrons in the atom, the hydrogen atom can be described by the Schrödinger equation and the Matrix Mechanics equation both derived in 1925. However, for hydrogen alone, in 1927 the Heitler–London theory was formulated which for the first time enabled the calculation of bonding properties of the hydrogen molecule H 2 based on quantum mechanical considerations. Specifically, Walter Heitler determined how to use Schrödinger's wave equation (1926) to show how two hydrogen atom wavefunctions join together, with plus, minus, and exchange terms, to form

5490-431: Is said to be represented by a resonance hybrid, it does not mean that electrons of the molecule are "resonating" or shifting back and forth between several sets of positions, each one represented by a Lewis structure. Rather, it means that the set of contributing structures represents an intermediate structure (a weighted average of the contributors), with a single, well-defined geometry and distribution of electrons. It

5612-456: Is small) parent molecule. This cation (an allylic cation) can be represented using resonance, as shown above. This observation of greater delocalization in less stable molecules is quite general. The excited states of conjugated dienes are stabilised more by conjugation than their ground states, causing them to become organic dyes. A well-studied example of delocalization that does not involve π electrons ( hyperconjugation ) can be observed in

5734-459: Is strict for the Period 2 elements Be, B, C, N, O, and F, as is a maximum of two for H and He and effectively for Li as well. The issue of expansion of the valence shell of third period and heavier main group elements is controversial. A Lewis structure in which a central atom has a valence electron count greater than eight traditionally implies the participation of d orbitals in bonding. However,

5856-468: Is the crystal structure , or arrangement, of the atoms. Another phase commonly encountered in the study of chemistry is the aqueous phase, which is the state of substances dissolved in aqueous solution (that is, in water). Less familiar phases include plasmas , Bose–Einstein condensates and fermionic condensates and the paramagnetic and ferromagnetic phases of magnetic materials. While most familiar phases deal with three-dimensional systems, it

5978-458: Is the quantum mechanical model . Traditional chemistry starts with the study of elementary particles , atoms , molecules , substances , metals , crystals and other aggregates of matter . Matter can be studied in solid, liquid, gas and plasma states , in isolation or in combination. The interactions, reactions and transformations that are studied in chemistry are usually the result of interactions between atoms, leading to rearrangements of

6100-485: Is the condition of maximum overlap, which leads to the formation of the strongest possible bonds. This theory is used to explain the covalent bond formation in many molecules. For example, in the case of the F 2 molecule, the F−F bond is formed by the overlap of p z orbitals of the two F atoms, each containing an unpaired electron. Since the nature of the overlapping orbitals are different in H 2 and F 2 molecules,

6222-507: Is the probability of a molecule to have energy greater than or equal to E at the given temperature T. This exponential dependence of a reaction rate on temperature is known as the Arrhenius equation . The activation energy necessary for a chemical reaction to occur can be in the form of heat, light, electricity or mechanical force in the form of ultrasound . A related concept free energy , which also incorporates entropy considerations,

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6344-457: Is to be distinguished from isomerism . Isomers are molecules with the same chemical formula but are distinct chemical species with different arrangements of atomic nuclei in space. Resonance contributors of a molecule, on the other hand, can only differ in the way electrons are formally assigned to atoms in the Lewis structure depictions of the molecule. Specifically, when a molecular structure

6466-405: Is used to indicate that A and B are contributing forms of a single chemical species (as opposed to an equilibrium arrow, e.g., A ↽ − − ⇀ B {\displaystyle {\ce {A <=> B}}} ; see below for details on usage). A non-chemical analogy is illustrative: one can describe

6588-526: Is useful in identifying periodic trends . A compound is a pure chemical substance composed of more than one element. The properties of a compound bear little similarity to those of its elements. The standard nomenclature of compounds is set by the International Union of Pure and Applied Chemistry (IUPAC). Organic compounds are named according to the organic nomenclature system. The names for inorganic compounds are created according to

6710-509: The Woodward–Hoffmann rules often come in handy while proposing a mechanism for a chemical reaction. According to the IUPAC gold book, a chemical reaction is "a process that results in the interconversion of chemical species." Accordingly, a chemical reaction may be an elementary reaction or a stepwise reaction . An additional caveat is made, in that this definition includes cases where

6832-455: The chemical bonds which hold atoms together. Such behaviors are studied in a chemistry laboratory . The chemistry laboratory stereotypically uses various forms of laboratory glassware . However glassware is not central to chemistry, and a great deal of experimental (as well as applied/industrial) chemistry is done without it. A chemical reaction is a transformation of some substances into one or more different substances. The basis of such

6954-493: The chemical industry . The word chemistry comes from a modification during the Renaissance of the word alchemy , which referred to an earlier set of practices that encompassed elements of chemistry, metallurgy , philosophy , astrology , astronomy , mysticism , and medicine . Alchemy is often associated with the quest to turn lead or other base metals into gold, though alchemists were also interested in many of

7076-469: The duet rule , and in this way they are reaching the electron configuration of the noble gas helium , which has two electrons in its outer shell. Similarly, theories from classical physics can be used to predict many ionic structures. With more complicated compounds, such as metal complexes , valence bond theory is less applicable and alternative approaches, such as the molecular orbital theory, are generally used. See diagram on electronic orbitals. In

7198-414: The formal apportionment of electrons to the atoms, and not in the actual physically and chemically significant electron or spin density. While contributing structures may differ in formal bond orders and in formal charge assignments, all contributing structures must have the same number of valence electrons and the same spin multiplicity . Because electron delocalization lowers the potential energy of

7320-510: The inorganic nomenclature system. When a compound has more than one component, then they are divided into two classes, the electropositive and the electronegative components. In addition the Chemical Abstracts Service has devised a method to index chemical substances. In this scheme each chemical substance is identifiable by a number known as its CAS registry number . A molecule is the smallest indivisible portion of

7442-500: The interconversion of conformers is experimentally observable. Such detectable chemical reactions normally involve sets of molecular entities as indicated by this definition, but it is often conceptually convenient to use the term also for changes involving single molecular entities (i.e. 'microscopic chemical events'). An ion is a charged species, an atom or a molecule, that has lost or gained one or more electrons. When an atom loses an electron and thus has more protons than electrons,

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7564-529: The intermolecular forces of a substance are such that the energy of the surroundings is not sufficient to overcome them, it occurs in a more ordered phase like liquid or solid as is the case with water (H 2 O); a liquid at room temperature because its molecules are bound by hydrogen bonds . Whereas hydrogen sulfide (H 2 S) is a gas at room temperature and standard pressure, as its molecules are bound by weaker dipole–dipole interactions . The transfer of energy from one chemical substance to another depends on

7686-529: The octet rule , possibly bearing formal charges , and connected by bonds of positive integer order, is sufficient for describing the chemical bonding and rationalizing experimentally determined molecular properties like bond lengths , angles , and dipole moment . However, in some cases, more than one Lewis structure could be drawn, and experimental properties are inconsistent with any one structure. In order to address this type of situation, several contributing structures are considered together as an average, and

7808-438: The size of energy quanta emitted from one substance. However, heat energy is often transferred more easily from almost any substance to another because the phonons responsible for vibrational and rotational energy levels in a substance have much less energy than photons invoked for the electronic energy transfer. Thus, because vibrational and rotational energy levels are more closely spaced than electronic energy levels, heat

7930-585: The π -electrons. Valence bond treatments are restricted to relatively small molecules, largely due to the lack of orthogonality between valence bond orbitals and between valence bond structures, while molecular orbitals are orthogonal. Additionally, valence bond theory cannot explain electronic transitions and spectroscopic properties as effectively as MO theory. Furthermore, while VBT employs hybridization to explain bonding, it can oversimplify complex bonding situations, limiting its applicability in more intricate molecular geometries such as transition metal compounds. On

8052-420: The 1980s, the more difficult problems, of implementing valence bond theory into computer programs, have been solved largely, and valence bond theory has seen a resurgence. According to this theory a covalent bond is formed between two atoms by the overlap of half filled valence atomic orbitals of each atom containing one unpaired electron. A valence bond structure is similar to a Lewis structure , but where

8174-522: The 2 p z orbital of F, each containing an unpaired electron. Mutual sharing of electrons between H and F results in a covalent bond in HF. Chemistry Chemistry is the scientific study of the properties and behavior of matter . It is a physical science within the natural sciences that studies the chemical elements that make up matter and compounds made of atoms , molecules and ions : their composition, structure, properties, behavior and

8296-533: The 2p z AOs on each of the 6 C atoms. Thus, each π MO is delocalized over the whole benzene molecule and any electron occupying an MO will be delocalized over the whole molecule. This MO interpretation has inspired the picture of the benzene ring as a hexagon with a circle inside. When describing benzene, the VB concept of localized σ bonds and the MO concept of delocalized π orbitals are frequently combined in elementary chemistry courses. The contributing structures in

8418-527: The Lewis description of the bonding in NO 2 is reconciled with the experimental fact that the anion has equivalent N–O bonds. The resonance hybrid represents the actual molecule as the "average" of the contributing structures, with bond lengths and partial charges taking on intermediate values compared to those expected for the individual Lewis structures of the contributors, were they to exist as "real" chemical entities. The contributing structures differ only in

8540-500: The VB model are particularly useful in predicting the effect of substituents on π systems such as benzene. They lead to the models of contributing structures for an electron-withdrawing group and electron-releasing group on benzene. The utility of MO theory is that a quantitative indication of the charge from the π system on an atom can be obtained from the squares of the weighting coefficient c i on atom C i . Charge q i  ≈  c i . The reason for squaring

8662-515: The Valence Shell Electron Pair Repulsion model ( VSEPR ), and the concept of oxidation number can be used to explain molecular structure and composition. An ionic bond is formed when a metal loses one or more of its electrons, becoming a positively charged cation, and the electrons are then gained by the non-metal atom, becoming a negatively charged anion. The two oppositely charged ions attract one another, and

8784-523: The actual molecule more than another (in the sense of energy and stability). Structures with a low value of potential energy are more stable than those with high values and resemble the actual structure more. The most stable contributing structures are called major contributors . Energetically unfavourable and therefore less favorable structures are minor contributors . With rules listed in rough order of diminishing importance, major contributors are generally structures that A maximum of eight valence electrons

8906-494: The atom is a positively charged ion or cation . When an atom gains an electron and thus has more electrons than protons, the atom is a negatively charged ion or anion . Cations and anions can form a crystalline lattice of neutral salts , such as the Na and Cl ions forming sodium chloride , or NaCl. Examples of polyatomic ions that do not split up during acid–base reactions are hydroxide (OH ) and phosphate (PO 4 ). Plasma

9028-448: The bible of modern chemistry. This book helped experimental chemists to understand the impact of quantum theory on chemistry. However, the later edition in 1959 failed to adequately address the problems that appeared to be better understood by molecular orbital theory. The impact of valence theory declined during the 1960s and 1970s as molecular orbital theory grew in usefulness as it was implemented in large digital computer programs. Since

9150-531: The bond region. Valence bond theory views bonds as weakly coupled orbitals (small overlap). Valence bond theory is typically easier to employ in ground state molecules. The core orbitals and electrons remain essentially unchanged during the formation of bonds. The overlapping atomic orbitals can differ. The two types of overlapping orbitals are sigma and pi. Sigma bonds occur when the orbitals of two shared electrons overlap head-to-head. Pi bonds occur when two orbitals overlap when they are parallel. For example,

9272-406: The bond strength and bond lengths differ between H 2 and F 2 molecules. In methane (CH 4 ), the carbon atom undergoes sp hybridization, allowing it to form four equivalent sigma bonds with hydrogen atoms, resulting in a tetrahedral geometry. Hybridization also explains the equal C-H bond strengths. In an HF molecule the covalent bond is formed by the overlap of the 1 s orbital of H and

9394-411: The changes they undergo during reactions with other substances . Chemistry also addresses the nature of chemical bonds in chemical compounds . In the scope of its subject, chemistry occupies an intermediate position between physics and biology . It is sometimes called the central science because it provides a foundation for understanding both basic and applied scientific disciplines at

9516-452: The characteristics of a real animal, the narwhal , in terms of the characteristics of two mythical creatures: the unicorn , a creature with a single horn on its head, and the leviathan , a large, whale-like creature. The narwhal is not a creature that goes back and forth between being a unicorn and being a leviathan, nor do the unicorn and leviathan have any physical existence outside the collective human imagination. Nevertheless, describing

9638-525: The coefficient is that if an electron is described by an AO, then the square of the AO gives the electron density . The AOs are adjusted ( normalized ) so that AO  = 1, and q i  ≈ ( c i AO i ) ≈  c i . In benzene, q i  = 1 on each C atom. With an electron-withdrawing group q i  < 1 on the ortho and para C atoms and q i  > 1 for an electron-releasing group . Weighting of

9760-489: The concept of quantum superposition and confused it with tautomerism . Pauling and Wheland themselves characterized Erich Hückel 's approach as "cumbersome" at the time, and his lack of communication skills contributed: when Robert Robinson sent him a friendly request, he responded arrogantly that he is not interested in organic chemistry. In the Soviet Union, resonance theory – especially as developed by Pauling –

9882-489: The consensus opinion is that while they may make a marginal contribution, the participation of d orbitals is unimportant, and the bonding of so-called hypervalent molecules are, for the most part, better explained by charge-separated contributing forms that depict three-center four-electron bonding . Nevertheless, by tradition, expanded octet structures are still commonly drawn for functional groups like sulfoxides , sulfones , and phosphorus ylides , for example. Regarded as

10004-412: The context of chemistry, energy is an attribute of a substance as a consequence of its atomic , molecular or aggregate structure . Since a chemical transformation is accompanied by a change in one or more of these kinds of structures, it is invariably accompanied by an increase or decrease of energy of the substances involved. Some energy is transferred between the surroundings and the reactants of

10126-616: The contributing structures in terms of their contribution to the overall structure can be calculated in multiple ways, using "Ab initio" methods derived from Valence Bond theory, or else from the Natural Bond Orbitals (NBO) approaches of Weinhold NBO5 Archived 2008-02-08 at the Wayback Machine , or finally from empirical calculations based on the Hückel method. A Hückel method-based software for teaching resonance

10248-437: The contributing structures. In molecular orbital theory , the main alternative to valence bond theory , the molecular orbitals (MOs) are approximated as sums of all the atomic orbitals (AOs) on all the atoms; there are as many MOs as AOs. Each AO i has a weighting coefficient c i that indicates the AO's contribution to a particular MO. For example, in benzene, the MO model gives us 6 π MOs which are combinations of

10370-474: The coupling produces two modes, one of which is lower in frequency than either of the uncoupled vibrations; quantum mechanically, this lower frequency is interpreted as a lower energy. Linus Pauling used this mechanism to explain the partial valence of molecules in 1928, and developed it further in a series of papers in 1931-1933. The alternative term mesomerism popular in German and French publications with

10492-436: The electron-deficiency on each atom and stabilizes the molecule. Below are the contributing structures of an individual 3c-2e bond in diborane. Often, reactive intermediates such as carbocations and free radicals have more delocalized structure than their parent reactants, giving rise to unexpected products. The classical example is allylic rearrangement . When 1 mole of HCl adds to 1 mole of 1,3-butadiene, in addition to

10614-586: The electrons delocalize to a more stable configuration, resulting in a reaction that happens at a different location. An example is the Friedel–Crafts alkylation of benzene with 1-chloro-2-methylpropane; the carbocation rearranges to a tert - butyl group stabilized by hyperconjugation , a particular form of delocalization. Comparing the two contributing structures of benzene, all single and double bonds are interchanged. Bond lengths can be measured, for example using X-ray diffraction . The average length of

10736-429: The energies and distributions characterize the availability of an electron to bond to another atom. The chemical bond can be a covalent bond , an ionic bond , a hydrogen bond or just because of Van der Waals force . Each of these kinds of bonds is ascribed to some potential. These potentials create the interactions which hold atoms together in molecules or crystals . In many simple compounds, valence bond theory ,

10858-444: The formation or dissociation of molecules, that is, molecules breaking apart to form two or more molecules or rearrangement of atoms within or across molecules. Chemical reactions usually involve the making or breaking of chemical bonds. Oxidation, reduction , dissociation , acid–base neutralization and molecular rearrangement are some examples of common chemical reactions. A chemical reaction can be symbolically depicted through

10980-460: The ground state, while the antisymmetric combination gives the first excited state , as shown. In general, the superposition is written with undetermined coefficients, which are then variationally optimized to find the lowest possible energy for the given set of basis wave functions. When more contributing structures are included, the molecular wave function becomes more accurate and more excited states can be derived from different combinations of

11102-461: The high number of oxygen atoms (four) and high electronegativity of the central chlorine atom leads to perchloric acid being one of the strongest known acids with a p K a value of −10. The extent of charge delocalization in an anion can be quantitatively expressed via the WAPS (weighted average positive sigma) parameter parameter and an analogous WANS (weighted average negative sigma) parameter

11224-399: The hybrid structure is drawn on the right with a charge of − 1 ⁄ 2 on both oxygen atoms and partial double bonds with a full and dashed line and bond order 1 + 1 ⁄ 2 . For hypervalent molecules , the rationalization described above can be applied to generate contributing structures to explain the bonding in such molecules. Shown below are the contributing structures of

11346-411: The idea that there is only one type of carbon-carbon bond, intermediate between a single and a double bond. The mechanism of resonance was introduced into quantum mechanics by Werner Heisenberg in 1926 in a discussion of the quantum states of the helium atom. He compared the structure of the helium atom with the classical system of resonating coupled harmonic oscillators . In the classical system,

11468-445: The ionic bond is the electrostatic force of attraction between them. For example, sodium (Na), a metal, loses one electron to become an Na cation while chlorine (Cl), a non-metal, gains this electron to become Cl . The ions are held together due to electrostatic attraction, and that compound sodium chloride (NaCl), or common table salt, is formed. In a covalent bond, one or more pairs of valence electrons are shared by two atoms:

11590-420: The last step, the hydrogenation of cyclohexene. In benzene, however, 23.4 kJ (5.6 kcal) are needed to hydrogenate one mole of double bonds. The difference, being 143.1 kJ (34.2 kcal), is the empirical resonance energy of benzene. Because 1,3-cyclohexadiene also has a small delocalization energy (7.6 kJ or 1.8 kcal/mol) the net resonance energy, relative to the localized cyclohexatriene,

11712-448: The lower the reactivity) and, specifically, the acid strength of their conjugate acids. As a general rule, the better delocalized is the charge in an anion the stronger is its conjugate acid . For example, the negative charge in perchlorate anion ( ClO 4 ) is evenly distributed among the symmetrically oriented oxygen atoms (and a part of it is also kept by the central chlorine atom). This excellent charge delocalization combined with

11834-622: The lowest potential energy is called the resonance energy or delocalization energy. The magnitude of the resonance energy depends on assumptions made about the hypothetical "non-stabilized" species and the computational methods used and does not represent a measurable physical quantity, although comparisons of resonance energies computed under similar assumptions and conditions may be chemically meaningful. Molecules with an extended π system such as linear polyenes and polyaromatic compounds are well described by resonance hybrids as well as by delocalised orbitals in molecular orbital theory . Resonance

11956-473: The main characteristics of a molecule is its geometry often called its structure . While the structure of diatomic, triatomic or tetra-atomic molecules may be trivial, (linear, angular pyramidal etc.) the structure of polyatomic molecules, that are constituted of more than six atoms (of several elements) can be crucial for its chemical nature. A chemical substance is a kind of matter with a definite composition and set of properties . A collection of substances

12078-412: The molecule is said to be represented by a resonance hybrid in which several Lewis structures are used collectively to describe its true structure. For instance, in NO 2 , nitrite anion, the two N–O bond lengths are equal, even though no single Lewis structure has two N–O bonds with the same formal bond order . However, its measured structure is consistent with a description as a resonance hybrid of

12200-425: The narwhal in terms of these imaginary creatures provides a reasonably good description of its physical characteristics. Due to confusion with the physical meaning of the word resonance , as no entities actually physically "resonate", it has been suggested that the term resonance be abandoned in favor of delocalization and resonance energy abandoned in favor of delocalization energy . A resonance structure becomes

12322-456: The non-classical 2-Norbornyl cation Another example is methanium ( CH 5 ). These can be viewed as containing three-center two-electron bonds and are represented either by contributing structures involving rearrangement of σ electrons or by a special notation, a Y that has the three nuclei at its three points. Delocalized electrons are important for several reasons; a major one is that an expected chemical reaction may not occur because

12444-456: The ordinarily expected product 3-chloro-1-butene, we also find 1-chloro-2-butene. Isotope labelling experiments have shown that what happens here is that the additional double bond shifts from 1,2 position to 2,3 position in some of the product. This and other evidence (such as NMR in superacid solutions) shows that the intermediate carbocation must have a highly delocalized structure, different from its mostly classical (delocalization exists but

12566-399: The other hand, valence bond theory provides a much more accurate picture of the reorganization of electronic charge that takes place when bonds are broken and formed during the course of a chemical reaction. In particular, valence bond theory correctly predicts the dissociation of homonuclear diatomic molecules into separate atoms, while simple molecular orbital theory predicts dissociation into

12688-419: The other isolated chemical elements consist of either molecules or networks of atoms bonded to each other in some way. Identifiable molecules compose familiar substances such as water, air, and many organic compounds like alcohol, sugar, gasoline, and the various pharmaceuticals . However, not all substances or chemical compounds consist of discrete molecules, and indeed most of the solid substances that make up

12810-444: The proper character for the bonding is called hybridization . Valence bond theory complements molecular orbital theory , which does not adhere to the valence bond idea that electron pairs are localized between two specific atoms in a molecule but that they are distributed in sets of molecular orbitals which can extend over the entire molecule. Although both theories describe chemical bonding, molecular orbital theory generally offers

12932-631: The questions of modern chemistry. The modern word alchemy in turn is derived from the Arabic word al-kīmīā ( الكیمیاء ). This may have Egyptian origins since al-kīmīā is derived from the Ancient Greek χημία , which is in turn derived from the word Kemet , which is the ancient name of Egypt in the Egyptian language. Alternately, al-kīmīā may derive from χημεία 'cast together'. The current model of atomic structure

13054-478: The reaction absorbs heat from the surroundings. Chemical reactions are invariably not possible unless the reactants surmount an energy barrier known as the activation energy . The speed of a chemical reaction (at given temperature T) is related to the activation energy E, by the Boltzmann's population factor e − E / k T {\displaystyle e^{-E/kT}} – that

13176-433: The reaction in the form of heat or light ; thus the products of a reaction may have more or less energy than the reactants. A reaction is said to be exergonic if the final state is lower on the energy scale than the initial state; in the case of endergonic reactions the situation is the reverse. A reaction is said to be exothermic if the reaction releases heat to the surroundings; in the case of endothermic reactions ,

13298-544: The resulting electrically neutral group of bonded atoms is termed a molecule . Atoms will share valence electrons in such a way as to create a noble gas electron configuration (eight electrons in their outermost shell) for each atom. Atoms that tend to combine in such a way that they each have eight electrons in their valence shell are said to follow the octet rule . However, some elements like hydrogen and lithium need only two electrons in their outermost shell to attain this stable configuration; these atoms are said to follow

13420-403: The rules of quantum mechanics , which require quantization of energy of a bound system. The atoms/molecules in a higher energy state are said to be excited. The molecules/atoms of substance in an excited energy state are often much more reactive; that is, more amenable to chemical reactions. The phase of a substance is invariably determined by its energy and the energy of its surroundings. When

13542-474: The same mass number; atoms of an element which have different mass numbers are known as isotopes . For example, all atoms with 6 protons in their nuclei are atoms of the chemical element carbon , but atoms of carbon may have mass numbers of 12 or 13. The standard presentation of the chemical elements is in the periodic table , which orders elements by atomic number. The periodic table is arranged in groups , or columns, and periods , or rows. The periodic table

13664-602: The same meaning was introduced by C. K. Ingold in 1938, but did not catch on in the English literature. The current concept of mesomeric effect has taken on a related but different meaning. The double headed arrow was introduced by the German chemist Fritz Arndt who preferred the German phrase zwischenstufe or intermediate stage . Resonance theory dominated over competing Hückel method for two decades thanks to being relatively easier to understand for chemists without fundamental physics background, even if they couldn't grasp

13786-417: The sixfold symmetry of the real molecule. The theory constructs the actual wave function as a linear superposition of the wave functions representing the two structures. As both Kekulé structures have equal energy, they are equal contributors to the overall structure – the superposition is an equally weighted average, or a 1:1 linear combination of the two in the case of benzene. The symmetric combination gives

13908-645: The solid crust, mantle, and core of the Earth are chemical compounds without molecules. These other types of substances, such as ionic compounds and network solids , are organized in such a way as to lack the existence of identifiable molecules per se . Instead, these substances are discussed in terms of formula units or unit cells as the smallest repeating structure within the substance. Examples of such substances are mineral salts (such as table salt ), solids like carbon and diamond, metals, and familiar silica and silicate minerals such as quartz and granite. One of

14030-537: The start of "modern valence bond theory", as contrasted with older valence bond theories, which are essentially electronic theories of valence couched in pre-wave-mechanical terms. Linus Pauling published in 1931 his landmark paper on valence bond theory: "On the Nature of the Chemical Bond". Building on this article, Pauling's 1939 textbook: On the Nature of the Chemical Bond would become what some have called

14152-695: The structure of the system, instead of changing the bulk conditions. Sometimes the distinction between phases can be continuous instead of having a discrete boundary' in this case the matter is considered to be in a supercritical state. When three states meet based on the conditions, it is known as a triple point and since this is invariant, it is a convenient way to define a set of conditions. The most familiar examples of phases are solids , liquids , and gases . Many substances exhibit multiple solid phases. For example, there are three phases of solid iron (alpha, gamma, and delta) that vary based on temperature and pressure. A principal difference between solid phases

14274-421: The study of chemistry; some of them are: In chemistry, matter is defined as anything that has rest mass and volume (it takes up space) and is made up of particles . The particles that make up matter have rest mass as well – not all particles have rest mass, such as the photon . Matter can be a pure chemical substance or a mixture of substances. The atom is the basic unit of chemistry. It consists of

14396-564: The true delocalized structure into that of the most stable contributing structure. The empirical resonance energy can be estimated by comparing the enthalpy change of hydrogenation of the real substance with that estimated for the contributing structure. The complete hydrogenation of benzene to cyclohexane via 1,3-cyclohexadiene and cyclohexene is exothermic ; 1 mole of benzene delivers 208.4 kJ (49.8 kcal). [REDACTED] Hydrogenation of one mole of double bonds delivers 119.7 kJ (28.6 kcal), as can be deduced from

14518-427: The two major contributing structures shown above: it has two equal N–O bonds of 125 pm, intermediate in length between a typical N–O single bond (145 pm in hydroxylamine , H 2 N–OH) and N–O double bond (115 pm in nitronium ion , [O=N=O] ). According to the contributing structures, each N–O bond is an average of a formal single and formal double bond, leading to a true bond order of 1.5. By virtue of this averaging,

14640-465: The valence electrons in their outer shell. In 1916, Kossel put forth his theory of the ionic chemical bond ( octet rule ), also independently advanced in the same year by Gilbert N. Lewis . Walther Kossel put forward a theory similar to Lewis' only his model assumed complete transfers of electrons between atoms, and was thus a model of ionic bonding . Both Lewis and Kossel structured their bonding models on that of Abegg's rule (1904). Although there

14762-404: The whole molecule. In 1916, G. N. Lewis proposed that a chemical bond forms by the interaction of two shared bonding electrons, with the representation of molecules as Lewis structures . The chemist Charles Rugeley Bury suggested in 1921 that eight and eighteen electrons in a shell form stable configurations. Bury proposed that the electron configurations in transitional elements depended upon

14884-632: Was attacked in the early 1950s as being contrary to the Marxist principles of dialectical materialism , and in June 1951 the Soviet Academy of Sciences under the leadership of Alexander Nesmeyanov convened a conference on the chemical structure of organic compounds, attended by 400 physicists, chemists, and philosophers, where "the pseudo-scientific essence of the theory of resonance was exposed and unmasked". One contributing structure may resemble

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