Electronegativity is a chemical residential property that explains the tendency of an atom or a functional team to tempt electrons toward itself.

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From: Principles and also Applications that Quantum Chemistry, 2016

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Robert J. Ouellette, J. David Rawn, in principles of organic Chemistry, 2015

Electronegativity

Electronegativity is a measure up of the attraction of an atom because that bonding electrons in molecules compared to that of other atoms. The electronegativity worths devised by Linus Pauling, one American chemist, room dimensionless amounts that selection from slightly much less than one for the alkali steels to a best of 4 for fluorine. Large electronegativity values show a stronger attraction because that electrons than little electronegativity values.


Electronegativities rise from left to right across the periodic table (Figure 1.3). Facets on the left that the periodic table have actually low electronegativities and are often dubbed electropositive elements. The stimulate of electronegativities F > O > N > C is crucial property that we will usage to explain the chemistry properties of organic compounds. Electronegativities to decrease from top to bottom in ~ a group of elements. The order of diminish electronegativities F > Cl > Br > I is an additional sequence that us will use to analyze the chemical and physical nature of organic compounds.



Figure 1.3. Electronegativity


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Characterization of chemistry Reactions


V.P. Gupta, in Principles and also Applications that Quantum Chemistry, 2016

12.14 Electronegativity and also Group Electronegativity

12.14.1 Electronegativity

Electronegativity is a chemical building that describes the tendency of an atom or a functional group to tempt electrons toward itself. The electronegativity of one atom is impacted by both its atomic number and the street that the valence electrons reside native the charged nuclei. The concept of electronegativity was an initial proposed by Pauli in 1932 as an explanation that the reality that the covalent bond in between two various atoms (AB) is stronger than would be supposed by acquisition the typical of the toughness of the AA and also BB bonds. This additional stabilization of the heteronuclear bond has actually been defined by the valence shortcut theory because of the donation of the ionic canonical forms to the bonding. Electronegativity is a loved one term and only the differences in the electronegativity space defined. Thus, Pauli proposed


(12.14.1)χA–χB=1/2(eV)−1/2,

where χA and χB are electronegativities of atom A and B and also D(AB), D(AA), and also D(BB) room the dissociation energies of bond AB, AA, and BB, respectively, express in units of electron volts (eV). The consist of of (eV)−1/2 expresses χA–χB as a dimensionless quantity. In stimulate to build an electronegativity scale, Pauli made decision hydrogen as recommendation as it develops covalent bonds through a large variety that elements and fixed that is electronegativity in ~ 2.1, which was later revised come 2.20. The atomic electronegativity range so created is well-known as Pauli's electronegativity scale. Electronegativity to be originally identified as one invariant residential property of atoms. Freshly several employees have argued that electronegativity of one atom counts upon its environment in the molecule. Thus, it depends on hybridization and also oxidation state the the element.


Mulliken <38> presented the ide of pure electronegativity and also proposed the the arithmetic mean of the very first ionization energy (I) and also the electron affinity (A) of an facet should be the measure up of the tendency of an atom to entice electrons.


(12.14.2)χ=(I+A)2

This expression based on the meanings of I and A, complies with from basic considerations. Thus, if X and also Y room two systems of interest, climate the problem that they have actually equal electronegativity is that the energy transforms for the reactions


X+Y→X++Y−

and


X+Y→X−+Y+

are equal.


(12.14.3)or,+=+

By definition, E(X+) − E(X) = I(X) is the ionization power of X and E(X) − E(X−) = A(X) is that is electron affinity.


Hence native equation (12.14.3), I(X) – A(Y) = I(Y) – A(X)


(12.14.4)or,for systemX=for systemY

It, therefore, renders sense to define electronegativity as (I + A). The aspect 1/2 in Eq. (12.14.2) was presented by Mulliken as he taken into consideration that χ together an arithmetic median of I and A is an quickly grasped concept.

12.14.2 group Electronegativity

Most that the attempts to develop electronegativity scale thought about electronegativity as an atomic property although indigenous Pauli's original definition, electronegativity is the strength or tendency of a team of atoms in a molecule to lure electrons to themselves. In necessary chemistry, electronegativity is associated an ext with various functional teams rather than with individual atoms. The electronegativity linked with functional teams is thus called group electronegativity or substituent electronegativity. Regardless of computational advancements and accessibility of big number of experimental data <39,40>, not numerous theoretical studies have been reported for team electronegativity. Relatively couple of methods have actually been proposed because that the evaluation of team electronegativity and most the them have been provided to evaluate only little subsets that the chemically amazing groups that atoms. It is usual to distinguish between the inductive effect and the resonance result which are explained as σ- and π-electronegativities, respectively. The quantization that the induction and also resonance effects is most regularly done by making use of Hammett's equations. Gupta et al. <12> learned isodesmic substituent stabilization energies in disubstituted ketenes family member to alkenes and also correlated lock with group electronegativities. They likewise analyzed the function of induction effect (F) and resonance impact (R) parameters that the substituent groups on charge distribution. A comparable study has been report by McAllister and Tidwell <41>. Boyd and Boyd <42,43> presented the bond vital point design for the decision of group electronegativities and reported worths for end 100 teams from ab initio calculations.


The electronegativity that a team A was calculated indigenous the properties associated with the AH bond an important point in AH. Because that example, the electronegativity χ the the methoxy group CH3O– was derived by determining the position of the five bond vital point in CH3OH and substituting the proper values in the complying with equations:


(12.14.5a)FA=rH/
(12.14.5b)χA=1.938FA−0.2502,

where FA is the electronegativity factor of the atom A, rH is the street from the bond crucial point to hydrogen nucleus, NA is the number of valence electrons of atom A (atom O in instance of CH3OH), ρ(rc) is the thickness at the bond an essential point, and rAH is the internuclear distance. The bond vital point and also the electron density at this point are identified by using the quantum theory of atoms-in-molecules. The group electronegativities of few of the crucial functional groups determined by Boyd and also Boyd <43> utilizing Eq. (12.14.5b) at the HF/6-31G∗//HF/6-31G∗ level room given in Table 12.4. In this table, the worths in the parenthesis be separated by a comma were obtained using HF/6-31G∗//experimental geometries and extended basis set//experimental geometry.


Table 12.4. Team electronegativities the some important functional groups


GroupxGroupx
CH32.55(2.55,2.56)CCl2F2.71(2.67, –)
CH2CH32.55(2.55,2.56)CCl32.70(2.66, –)
CH2CH2CH32.55(2.55, –)NH23.12(3.10,3.10)
CH2CH(CH3)22.54(2.53, –)NHCH33.13(3.11, –)
CH2CN2.58(2.56, –)NHCOH3.18(3.17, –)
CH2COH2.58(2.56, –)N(CH3)23.13(3.08, –)
CH2COO−2.52(–, –)NCO3.18(3.20,3.22)
CH2COOH2.58(–, –)NC3.26(3.26,3.30)
CH2COF2.59(2.58, –)NHNH23.13(3.12, –)
CH2CSH2.58(2.57, –)NNN3.15(3.23, –)
CH2COCl2.59(2.57, –)NO3.12(3.05,3.06)
CH(CH3)22.55(2.54, –)NHF3.19(–, –)
CH(CH2)22.57(2.56, –)NF23.25(–, –)
CHCH22.58(2.57,2.61)O-3.36(–, –)
CHCCH22.58(2.57, –)OH3.55(3.52,3.64)
CHCCO2.61(–, –)OH2+3.57(–, –)
CHCO2.58(2.57, –)OCH33.53(3.51,3.70)
C(CH3)32.55(2.53, –)OCN3.57(–, 3.73)
C6H52.58(2.58, –)OCOH3.56(3.50,3.65)
CCH2.66(2.65,2.66)OCOCH33.57(–, –)
CCF2.66(2.66, –)ONH23.58(3.54, –)
CCCl2.66(2.66, –)ONO3.55(3.49, –)
CH2NH22.55(2.54, –)OO.3.58(3.51, –)
CH2NHCH32.57(2.57, –)Of3.60(3.56, –)
CH2NO22.62(2.61, –)OPH2O3.54(–, –)
CHNH2.59(2.56, –)MgH1.30(1.31,1.33)
CHNN2.60(2.60, –)AlH21.60(1.61,1.62)
CN2.69(2.68,2.69)SiH1.87(1.87, –)
CNO2.69(2.73, –)SiH31.90(1.89,1.91)
CH2OH2.59(2.58,2.59)SiF1.88(–, –)
CH2OOCH32.61(2.60, –)SiF2H1.93(–, –)
COH2.60(2.58,2.60)SiCl1.89(1.85, –)
COCH32.59(2.59, –)SiClH21.91(1.90, –)
CONH22.61(2.60, –)SiCl2H1.93(1.92, –)
−CO2−2.49(–, –)SiCl31.95(1.94, –)
COOH2.63(2.62,2.66)PH22.17(2.16,2.17)
COOCH32.64(2.62, –)PH2O2.21(–, –)
CO2.57(2.54,2.57)PH(OH)O2.22(–, –)
COF2.67(2.65, –)P(OH)2O2.25(–, –)
COCl2.66(2.64, –)S-2.52(–, –)
CFH22.60(2.61,2.61)SH2.65(2.66,2.63)
CF2H2.65(2.64, –)SCH32.65(2.65, –)
CF32.71(2.68, –)SCN2.70(–, –)
CClH22.61(2.60, –)SSH2.68(2.67, –)
CClFH2.66(2.63, –)

1988, American chemistry Society.


Electronegativity is an index of the tendency of an atom to tempt electrons. It is proportional to the difference between an atom’s ionization potential and also its electron affinity. Linus Pauling inserted electronegativity worths on a range of slightly less than 1.0 because that alkali steels to a maximum of 4.0 for fluorine (Figure 1.3). The alkali metals and also alkaline earth metals often tend to shed an electron to gain an inert gas configuration. Thus, groups I and II contain the least electronegative atoms. In reality they are electropositive. ~ above the other finish of the scale, halogens, in team VII, tend to gain an electron to provide an inert gas configuration. Thus, we uncover that electronegativity boosts from left to right throughout the routine table. Electronegativity values increase in duration 2 in the bespeak C

Robert R. Crichton, in biological Inorganic Chemistry (Second Edition), 2012

Ionic Bonding

Electronegativity is the tendency of an atom to attract electrons in a molecule. Huge differences in electronegativity in between atoms in a offered molecule often cause the finish transfer of one electron native the unfilled outer shell that one atom to the unfilled shell of another. The result charged types (ions) are held together by electrostatic forces. Together bonds are extremely polarised and are referred to as ionic bonds. Ionic bonding is the simplest type of chemistry bonding encountered. NaCl, deserve to be composed as , the salt atom offering up one electron come resemble the steady neon atom, while the chlorine atom repurchase an extra electron to resemble the steady argon atom. MgCl2 and CoBr3 room other instances of ionic compounds.


Therald Moeller, ... Clyde Metz, in Chemistry: With not natural Qualitative Analysis, 1980

10.8 Electronegativity and oxidation state

Electronegativity values are advantageous in determining oxidation numbers in instances where it might otherwise be challenging to decision which atom has actually the positive and also which the negative oxidation number. In binary covalent compounds and also polyatomic ion such as PO43- or ClO4−, the more electronegative atom is arbitrarily assigned the an unfavorable oxidation number matching to the fee it would have if the were current as an ion. This leaves assignment that a confident oxidation number come the much less electronegative element. As a result, oxidation numbers stand for the loved one electronegativities that atoms and also the direction that the polarity that a bond. Because that example, in iodine trichloride, ICl3, chlorine is assigned the -1 oxidation number due to the fact that it is the more electronegative the the two atoms. To attain the essential neutrality, iodine is assigned one oxidation number of + 3 in this compound.


where a, b, c room the number of atom that a given facet (A, B, C, etc.).

As an applications of the electronegativity equalization rule (EEP), Parr and Pearson <18> obtained Eqs. (26) and (27) to measure the quantity of charge deliver ΔN and the energy change ΔE connected with the development of A:B facility from acid A and base:B.

These expressions are very useful in understanding the acid-base reaction mechanism. That is vital to keep in mind that the electronegativity distinction drives the electron transfer whereas the hardness sum offers a resistance come it. Therefore both χ and also η space to be taken into consideration in assessing these processes.


The molecular Electronegativity distance Vector (MEDV) is a vectorial descriptor, which includes 91 terms that contain information around the relative electronegativities.252 The electronegativities are stood for by the amendment E-state indices and topological distances between each possible pair the 13 atom types. In the an initial step the the MEDV descriptor calculation, one needs to assign each atom in the molecule to one of the abovementioned atom types. The atom species are based on the many frequently occurring atoms in organic molecules and also the variety of bonded nonhydrogen atoms (vertex degree).230 The solitary molecular descriptor have the right to be calculated through this equation:


(25)huv=∑i∈u∑j∈vSi*·Sj*dij2,u,v=1,2,…,13,

where u and v are the atom types, S* to represent the amendment E-state index, dij is the topological distance in between vi and also vj vertices.


There is additionally an extension of MEDV-13, referred to as Molecular Holographic street Vector (MHDV).253 the is developed to describe much more specific molecule structures prefer peptide sequences, which contain heteroatoms and also multiple bonds.


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E. Negishi, in comprehensive Organometallic Chemistry, 1982

45.6.2.1 Electronegativity and Size

The electronegativity the boron has been estimated at 2.0,15 which is fairly close to the equivalent value of 2.5 because that carbon15 and is higher than those of the alkali and also alkaline planet metals, e.g. lithium, and those of essentially all change metals. Making use of an empirical equation correlating shortcut ionicity and difference in electronegativity propose by Pauling,15a the boron–carbon bond might be approximated to be about 90% covalent and only 10% ionic.

The boron–carbon bond length in trimethylborane16 is 1.56 Å, which need to be contrasted with the carbon–carbon bond size of 1.54 Å because that ethane17 and the aluminum–carbon link lengths of 1.97 Å (terminal) and also 2.14 Å (bridging) for dimeric trimethylalane.18

The short ionicity and the reasonably short shortcut length incorporate to contribute to the truth that the bonding electrons of the boron–carbon shortcut of organoboranes often tend to be extremely inaccessible to external electrophilic reagents. In various other words, organoboranes, uneven organolithium and Grignard reagents, room of short nucleophilicity.


L.H. Hall, ... L.M. Hall, in substantial Medicinal Chemistry II, 2007

4.22.2.2 Valence State Electronegativity

Valence state electronegativity (VSE) is identified as the capacity of one atom, in the valence state, to lure electron density to itself from neighboring atoms v the sigma bonding network the the molecule. An experimental measure of VSE has actually been occurred by Hans Jaffeé and also co-workers,18 based upon the Mulliken meaning of electronegativity.19 In the Mulliken definition, electronegativity is based upon the median of the ionization potential (Ip) and also electron affinity (EA) because that an atom. Hinze and Jaffeé extended the Mulliken an interpretation to valence claims by making use of Ip and EA data taken from digital spectroscopic data because that atomic orbitals; they occurred numerical worths for valence states. This approach permits the review of XMJ values for miscellaneous hybrid valence states.


The Mulliken–Jaffeé valence state electronegativity worths (XMJ) characterize an atom in its number of valence says unlike the much less sensitive Pauling worths which are the exact same for every valence states. Because that example, the Pauling worth for carbon is the exact same for carbons atoms in ethane, ethylene, and acetylene.20 However, the XMJ worths are various for these three hydrocarbons, equivalent to the enhancing acidity of the hydrogen atoms in these 3 molecules. The larger valence state electronegativity because that the acetylenic carbon correlates through the better acidity of the hydrogen atom in acetylene and also the better polarity that the C–H bond. Similar information is residents in the XMJ worths for all atoms in your valence state (see Table 3).


Table 3. Delta values and experimental valence state electronegativity values


GroupNδδvδv– δXMJ (eV)a
C(sp3)21107.98
C(sp2)22318.79
C(sp)224210.39
N(sp3)235211.54
N(sp2)225312.87
N(sp)215415.68
O(sp3)226415.25
O(sp2)216517.07
F(sp3)217617.63
Si(sp3)31107.30
Si(sp2)32317.90
P(sp3)33528.90
S(sp3)326410.14
S(sp2)316510.88
Cl(sp3)317611.84
>As–43528.26
–Se–42649.08
–Br41769.90
–I51769.02

ar2=0.965, s=0.55, n=19. See message for discussion.

In the breakthrough of the E-state formalism, we integrated valence state digital information due to the fact that it characterizes the necessary molecule effectively. Because that this reason, we embraced the Mulliken–Jaffeé valence state electronegativity values as a recommendation for atom properties the are necessary to electron density. The an initial step in this advancement is the detect of a relationship between XMJ and also molecular framework information. The linkage can be taken from the point of view of the source of electronegativity in a molecule. This impact arises from the efficient nuclear charge on the atom, that is, the attraction the electrons developing from unshielded protons in the cell core of the atom.5,17

As a referral point, us can consider that a carbon atom in an sp3 valence state has four valence electron in 4 sigma orbitals. Each valence electron in that is sigma orbital, as part of the sigma bond, properly shields one atom proton. The efficient nuclear charge for this sp3 carbon is taken to be zero and the equivalent VSE is additionally taken to be zero.

For one sp2 carbon atom, one valence electron is in a pi orbital and also three space in sigma orbitals. In this case, the pi electron, who electron density is primarily outside of the sigma bonding orbital, does no effectively display the core charge. The reliable nuclear charge might be taken together one. The resulting electronegativity the the sp2 carbon is higher than because that the sp3 carbon. Following the very same line of argument, the effective nuclear charge for an acetylenic carbon (sp) is much greater than the of the sp2 carbon, based on two unshielded protons. Much more generally, for carbon valence state electronegativity, the complying with ranking is it was observed in properties and also described through valence state electronegativity:

*
C–H>
*
CH2>–CH3.

This strategy to valence state electronegativity can be generalized. The ineffective shielding occurs from the truth that pi and also lone pair electron density has short or zero probability along the heat of the shortcut axis. Together a result, pi and also lone pair electrons execute not display screen nuclear proton as properly as electron in sigma orbitals that are directed along the link axis.5,17

The count of the pi and also lone pair electrons on a external inspection atom deserve to be justified as a model of the electronegativity the its bound atom ~ above both experimental and theoretical grounds. The work-related of Slater has shown that the pi and lone pair electrons, being more from the core than the sigma electrons, an outcome in less shielding therefore a greater influence the the core on the sigma bonding electrons.23 This is the significance of electronegativity.

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The count of pi and lone pair electrons might be readily related come the efficient nuclear fee and, together a result, to the valence state electronegativity:


This expression, for second-row elements, includes the counting of the extrajacent electron on a sigma-bonded atom, the is, the count of pi (π) and also lone pair (n) electrons. For elements beyond fluorine, the added screening of the additional core electrons should be taken right into account. To include valence states for aspects beyond fluorine, the principal quantum variety of the valence electron (N) is included, as follows:


This expression was supplied in a design of VSE for 19 valence states, bring about a high correlation through XMJ v a traditional error of regression that approaches the known experimental errors, generally in the determination of electron affinity values.5,17 Table 3 mirrors values because that VSE because that 19 atom valence states with the corresponding values because that the delta values.