Electronegativity[1]It is a scale of the ability of an element's atoms to attract electrons in a compound.The greater the electronegativity of an element, the stronger the ability of its atoms to attract electrons in the compound.Also called relative electronegativity, electronegativity for short, also called electronegativity.The electronegativity is comprehensively consideredionization energyandElectron affinity, first byLinus Carl Pauling It was proposed in 1932 to measure the relative strength of the ability to attract electrons when forming chemical bonds between different atoms, usually expressed by the Greek letter χ.
Pauling defined electronegativity as "the scale of the ability of an element's atoms to attract electrons in a compound".The greater the electronegativity value of an element, the stronger the ability of its atom to attract electrons in the compound;Conversely, the weaker the ability of the corresponding atom to attract electrons in the compound（Noble gasExcept atoms).Physical concepts need to be quantified by the establishment of corresponding scales. Electronegativity can be scaled by a variety of experimental and theoretical methods.Electronegativity is similar to the nonmetallic nature of elements, but they are not completely equivalent.Electronegativity emphasizes the offset direction of shared electron pairs, while nonmetallic emphasizes the gain and loss of electrons.
Chinese name
Electronegativity
Foreign name
electronegativity
Discipline
Chemistry
Application
Study on properties of chemical substances
Definition
Scales of the ability of atoms of elements to attract electrons in compounds
Electronegativity, denoted by χ, refers to the attraction of shared electrons (orelectron density）Trends for[2]The higher the electronegativity, the stronger the ability of atoms to attract electrons.The electronegativity of an atom is affected by itsAtomic number, andvalence electronInfluence of distance from charged atomic nucleus.At the most basic level, electronegativity is determined byNuclear charge(The more protons an atom has, the greater the "pull" on electrons) and the number and location of other electrons in the atomic shell (the more electrons).The farther the valence electrons of atoms are away from the nucleus, thepositive chargeThe less attractive they are, not only because their distance from the nucleus increases, but also because other electrons in the lower energy core orbit will play a role in protecting the valence electrons from the positively charged nucleus.
Electronegativity can be used to estimate bond energy and chemical bond polarity.When forming compounds, elements with higher electronegativity values tend to be negative due to their strong attraction to bonding electronsValence；The lower electronegativity value shows positive valence.In the formation ofcovalent bondThe shared electron pair will shift to the atom with strong electronegativity and make the bond polar. The greater the electronegativity difference, the stronger the bond polarity.When the electronegativity of elements at both ends of the chemical bond is very different (for example, greater than 1.9), the bond formed is mainly ionic bond.
Development History
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The term "electronegativity" was proposed by J ö ns Jacob Berzelius in 1811[3], although the concept has been known before and is includedAvogadroMany chemists including[3]。Since Pauling first gave electronegativity values to some elements in 1932, the research process of more than 100 years can be divided into three stages.
The main feature of the first stage is to find a suitable method to calculate or measure atomic electronegativity, that is, the traditional electronegativity scale, of which the most representative are Pauling electronegativity scale, Mulliken electronegativity scale, Gordon electronegativity scale, Sanderson electronegativity scale and Allred Rockow electronegativity scale[4]。Because the electronegativity value cannot pass the experiment andquantum chemistryAccording to the method, in this stage of research, chemists mainly use various atomic property parameters to estimate electronegativity, such asionization energy、Electron affinity、Effective nuclear charge、Atomic radius、Polarizability, magnetic susceptibility, gradient field of atomic nucleus, atomic spectrum, magnetic resonance spectrum parameters, etc.The estimated electronegativity values are basically consistent with Pauling's electronegativity values, which is due to the periodicity of atomic property changes.
In the second stage, the group electronegativity and atomic charge are calculated based on the electronegativity equalization principle, and the hybridization mode of atoms and the different atomic charges are mainly discussedValence stateThe influence of different factors on atomic electronegativity.
In the third stage, Parr and its partners[5]Marked by the pioneering absolute electronegativityDeductive methodThe concept of absolute electronegativity is proposed, and the most representative scale is Allen electronegativity scale.Besides, Luo and Benson[6]Apply based onCovalent radiusThe new scale of electronegativity of the main group elements is determined by the open shell nuclear potential;Li Guosheng and Zheng Nengwu[7]The electronegativity scale is also calculated using the average nuclear potential of valence electrons.Yang Lixin[8]Reliable experimental data of valence layer ionization energy and valence bond through isolated atomsOrbital energy, usingEffective nuclear chargeA new scale of electronegativity of 90 elements in periodic table is established by numerical method.
computing method
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First of all, electronegativity is a relative value, so there is no unit.Moreover, there are many calculation methods of electronegativity (that is, different scales are used), so the electronegativity values of each method are different, so when using electronegativity values, they must be the same set of values for comparison.There are three representative electronegativity calculation methods:
① In 1932, LC. Pauline[9]An empirical fact has been found:Bond energyEA-BGreater than the bond energy between atoms of the same kindEA-AandEB-BThe average value of, so that the difference between them is Δ, then:
or
mostchemical compoundAre suitable for the formula of arithmetic mean[10]。
Pauling definition χASum χBThey are the electronegativity of elements A and B respectively, and the following semi empirical formula is proposed:
Where, Δ can be calculated from bond energy data (in kJ/mol), and 0.102 is obtained by fitting a large number of test data.
The semi empirical formula proposed by Pauling can calculate the absolute value of the electronegativity difference between two elements, and then manually specifyFluorine elementElectronegativity χ ofF=4.00, the relative electronegativity of other elements can be calculated.It is worth noting that according to the Pauling scale, the third largest element of electronegativity ischlorineThe fourth isnitrogen；But in fact, when nitrogen and chlorine formcovalent bondThe nitrogen shows negative valence while chlorine shows positive valence, that is, the experimental fact is inconsistent with the calculated value;One of the purposes of the later development of Millikan scale and Alei Rozhou scale is to solve this problem.
Pauling electronegativity scale is one of the most widely accepted electronegativity scales, but it does not haveUniversalityThe main reason is that Pauling's electronegativity scale has the following defects: first, its definition of N's electronegativity (3.04) is less than that of Cl's (3.16);Second, it can not provide the electronegativity of rare elements;Thirdly, the electronegativity of many transition metals is higher than that of Si, which violates the silicon rule (i.e., the electronegativity of metals should be lower than that of Si);Fourth, Pauling's electronegativity scale is not availablequantum mechanicsBased on the thermodynamic data of some elements.
The figure below shows Pauling's electronegativity data:
Pauling electronegativity meter
② 1934 RS. Mulliken calculated the absolute electronegativity from ionization potential and electron affinity energy, that is, the average value of ionization energy and electron affinity energy.
IIs ionization energy,AIs electronicAffinity(exothermic is positive, endothermic is negative)[11]。
Since this definition does not depend on any relative scale, it is also called absolute electronegativity[12], the unit is kJ/mol.However, it is more common to uselinear transformation Transform these absolute values into values similar to the more familiar Pauling values.Some elements do not have accurate ionization energy and electron affinity energy data, so their Mulliken electronegativity data cannot be obtained.In addition, Mulliken electronegativity scale is used to explain thePolarityIt also encountered some difficulties. For example, the Mulliken electronegativity data of H and N are 3.059 and 3.083 respectively, which are close to each other. Therefore, it is predicted that NHthreeThe molecule should be weak polar, but the experiment confirmed that NHthreeIt is a strong polar molecule.For another example, the Mulliken electronegativity data of C is 2.671, which is smaller than the electronegativity data of H, so CH is predicted accordinglyfourThe polarity direction of C-H bond in is more consistent with the measured CHfourThe polarity direction of C-H bond is contradictory.For another example, the electronegativity data of Cl is 3.535. If the electronegativity data is greater than N, actually NClthreeN in the molecule is negatively charged.
③ 1957 A50. Allred and ERochow used the electrostatic attraction of simple point charges to approximate the ability of atoms to attract electrons.They use Z*/rtwoPlotting Pauling's electronegativity, we can get a straight line whose equation is:
Among them,Z*Effective nuclear charge,rIs the radius.
The Allred Roche scale is also one of the three most commonly used electronegativity scales, which is commonly used to discuss theElectron distribution。However, when the number of cycles is greater than 4, the Allred Rockow electronegativity ratio of elements is significantly lower.Wen Yuankai, 1974-1975[13]The calculation method of shielding constant is improved, and the calculation formula of electronegativity is modified as follows:
The electronegativity data obtained is close to Pauling's electronegativity data.
Alei Rozhou electronegativity meter
④ L. 1989C.Allen[14-16]According to the spectral experimental dataground stateThe electronegativity of the main group elements is obtained based on the average unit electron energy of the valence shell electrons of free atoms, which is calledspectrumElectronegativity
：
amongNsandNpnamelyS trackandP OrbitNumber of electrons,εsandεpAre the s and p orbitals, respectivelyionization energy。Energy termεsandεpIt can be obtained from the weighted spectral dataNoble gasElectronegativity of.For elements in zone d,εdreplaceεp，εdIs the d orbital ionization energy;NdreplaceNp，NdIs the number of d orbital electrons.
Compared with the traditional electronegativity scale, the spectral electronegativity scale has a simple conceptual basis and obvious special properties;Spectral electronegativity includes the ionization energy of all valence electrons, as well as the occupied and non occupied orbitalenergy levelThe difference is related toOxidation number、coordination numberIt is irrelevant, reflecting the ability of atoms to attract and control electrons.
Spectral electronegativity has a good correlation with the basic properties of atoms in the periodic table, and can explain many problems in the periodic table.First, the spectral electronegativity value can accurately define metalloids (boron, silicon, germanium, arsenic, antimony, tellurium), that is, metalloids have a very narrow configuration energy range.The non-metallic elements have higher spectral electronegativity, which is on the right side of the metal nonmetal oblique boundary;The electronegativity of metal spectrum is low, which is on the left of the oblique boundary.The second is that the atomic radius of the elements in the p region gradually increases from top to bottom, the average value of the valence level gradually decreases, and the energy level spacing shrinks, which causes the energy levels of s, p, and d orbitals to be close to each other, making the bonding lose its directionality. The spectral electronegativity can characterize both the level and the energy levelenergy levelSpacing, so it can reflect the family elements from top to bottomMetallicityThe law of enhancement.Third, the size of spectral electronegativity can describe the nature of an element's own formation of molecular or solid bonds (from metal bonds → metalloid → covalent bonds).Three types ofChemical bond。The three vertices of the triangle represent three types of chemical bonds, where M represents metal bonds, C represents covalent bonds, and I represents ionic bonds.MC line representationperiodic tableThe M-I and C-I lines describe the properties of the bonds (ionic bonds or covalent bonds) of atoms forming binary compounds using spectral electronegativity differences.Fourthly, spectral electronegativity can also explain the oxidation state limitation of N, O, F, Cl, Br, He, Ne, Ar, Kr fluoride.The above elements have high spectral electronegativity in the periodic table, and the corresponding energy level difference is large. The unoccupied orbit is also not conducive to bonding, so some valence electrons are firmly bound, and it is difficult to participate in bonding directly.Fifthly, the spectral electronegativity can reflect the bonding effect of some transition metal elements.staytransition metalThe maximum value of the radial distribution of the (n-1) d orbital in the first half of the elements of the (n-1) d orbital tends to be closer to the atomic nucleus than the maximum value of the radial distribution of the ns orbital, that is, part of the d electrons approach the interior of the atom, and the number of the valence electrons decreases, resulting in the ionization energy of the (n-1) d orbital electrons is greater than the ionization energy of the ns orbital, and the spectral electronegativity decreases.
In addition, spectral electronegativity can also explain electronegativity and radius effect.Allen et al found that atomic radius can be expressed as a function of spectral electronegativity, that is
, b is 1-2, which is related to the atomic family, and r is the maximum radial distribution of valence electron outer layer.The d10 electrons of the fourth, fifth, and sixth periodic elements in the p region have a small shielding effect on the p orbital electrons, especially the s orbital electrons, so the spectral electronegativity increases and the electronegativity cross occurs.Spectral electronegativity reflects the average value of the energy level of the occupied electronic orbit, and can reflect the energy level difference between the occupied electronic orbit and the unoccupied electronic orbit. These are just two key parameters of the chemical bond, so the spectral electronegativity reflects the bonding situation.When explaining the chemical bonding of elements in the s and p regions, Allen's electronegativity scale is also more advantageous than other electronegativity scales.For example, Allen electronegativity data of H, S and I are 13.61, 15.31 and 13.95 respectively, which can explain HtwoThe polarity of S-H bond in S and I-H bond in HI.
Allen spectral electronegativity table
Electronegativity of common elements (Pauling scale)
Non metallic system: fluorine>oxygen>chlorine>nitrogen>bromine>iodine>sulfur>carbon
Metal system: aluminum>beryllium>magnesium>calcium>lithium>sodium>potassium
Generally speaking, the electronegativity of elements in the periodic table increases gradually from left to right;The electronegativity of elements gradually decreases from top to bottom of the periodic table.
1. WithatomThe electronegativity of elements is shown when the serial number increasesPeriodic change。
2. In the same period, the electronegativity of elements increases from left to right, and the electronegativity of elements in the same main family decreases from top to bottom.yesAccessory familyIn terms of, the electronegativity of elements of the same family also shows this trend.Therefore, the elements with high electronegativity are concentrated inperiodic table of ele mentsThe elements with low electronegativity are concentrated in the lower left corner.
3. The non-metallic elements with greater electronegativity are more active, and the metal elements with less electronegativity are more active.Fluorine has the largest electronegativity (4.0) and is the non-metal most easily involved in the reaction;Cesium, the element with the smallest electronegativity (0.79), is the most active metal.
(1) Judge the element'sMetallicityandNonmetallic。It is generally believed that non-metallic elements have electronegativity greater than 1.8, metallic elements have electronegativity less than 1.8, and elements around 1.8 have both metallic and non-metallic properties.
(2) Judge the positive and negative valence of elements in the compound.The element with small electronegativity value has weak ability to attract electrons in the compound, and the valence of the element is positive;Elements with high electronegativity have strong ability to attract electrons in compounds, and the valence of elements is negative.
(3) Determine the polarity and bond type of molecules.When nonmetallic elements with the same electronegativity combine to form compoundsnonpolar covalent bond , whose molecules areNonpolar molecule；It is generally believed that the formation between atoms of two elements with electronegativity difference less than 1.7Polar covalent bond, corresponding compounds areCovalent compound；When two elements with electronegativity difference greater than 1.7 combineIonic bond, the corresponding compound isIonic compound。
(4) In the periodic table“Diagonal rule”。Some main group elements in the periodic table have similar electronegativity and properties with the main group elements at the lower right.
(5) InterpretationNuclear magnetic resonance spectrumChemical shift in analysis[17]。The electron absorption induced effect of atoms with larger electronegativity will make the chemical shift move to the lower field.
