كهرسلبية

"Electronegative" redirects here. For the Nightfall EP, see Electronegative (EP).

Electrostatic potential map of a water molecule, where the oxygen atom has a more negative charge (red) than the positive (blue) hydrogen atoms

السالبية الكهربية أو الكهرسلبية هى مقياس لمقدرة الذرة أو الجزيء على جذب الإلكترونات في الروابط الكيميائية . وتعتمد نوعية الرابطة المتكونة إعتمادا كبيرا على الفرق في السالبية الكهربية بين الذرات الداخلة فيها . وتقوم الذرات المتشابهة في السالبية الكهربية " بسرقة " الإلكترونات من بعضها البعض والذى يرجع لما يسمة " مشاركة " وتكون رابطة تساهمية . ولكن لو كان هذا الفرق كبير سينتقل الإلكترون إلى أحد الذرات وتتكون رابطة أيونية . إضافة إلى ذلك في حالة أن أحد الذرات تقوم بسحب الإلكترونات بقوة أكبر قليلا من الأخرى فإنه تتكون رابطة تساهمية قطبية .

ويتم إستخدام مقياسين مشهورين للسالبية الكهربية، مقياس باولنج ( أقتُرِح في 1932 ) ومقياس مولكين ( تم إقتراحه عام 1934 ). كما يوجد إقتراح أخر يسمى مقياس ألريد-روشو.

On the most basic level, electronegativity is determined by factors like the nuclear charge (the more protons an atom has, the more "pull" it will have on electrons) and the number and location of other electrons in the atomic shells (the more electrons an atom has, the farther from the nucleus the valence electrons will be, and as a result, the less positive charge they will experience—both because of their increased distance from the nucleus and because the other electrons in the lower energy core orbitals will act to shield the valence electrons from the positively charged nucleus).

The term "electronegativity" was introduced by Jöns Jacob Berzelius in 1811, though the concept was known before that and was studied by many chemists including Avogadro. In spite of its long history, an accurate scale of electronegativity was not developed until 1932, when Linus Pauling proposed an electronegativity scale which depends on bond energies, as a development of valence bond theory. It has been shown to correlate with a number of other chemical properties. Electronegativity cannot be directly measured and must be calculated from other atomic or molecular properties. Several methods of calculation have been proposed, and although there may be small differences in the numerical values of the electronegativity, all methods show the same periodic trends between elements.

The most commonly used method of calculation is that originally proposed by Linus Pauling. This gives a dimensionless quantity, commonly referred to as the Pauling scale (χ_r), on a relative scale running from 0.79 to 3.98 (hydrogen = 2.20). When other methods of calculation are used, it is conventional (although not obligatory) to quote the results on a scale that covers the same range of numerical values: this is known as an electronegativity in Pauling units.

As it is usually calculated, electronegativity is not a property of an atom alone, but rather a property of an atom in a molecule. Even so, the electronegativity of an atom is strongly correlated with the first ionization energy. The electronegativity is slightly negatively correlated (for smaller electronegativity values) and rather strongly positively correlated (for most and larger electronegativity values) with the electron affinity. It is to be expected that the electronegativity of an element will vary with its chemical environment, but it is usually considered to be a transferable property, that is to say that similar values will be valid in a variety of situations.

Caesium is the least electronegative element (0.79); fluorine is the most (3.98).

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 طرق الحساب

 كهرسلبية پاولنگ

تم إقتراح مقياس باولنج عام 1932 . وفى هذا المقياس يكون عنصر الفلور هو أعلى العناصر في السالبية الكهربية حيث تبلغ 3.98 ، بينما أقل العناصر سالبية كهربية هو الفرانسيوم وله قيمة تبلغ 0.7 والعناصر الباقية تكون قيمها بين هاتين القيمتين . ويكون الهيدروجين له قيمة سالبية كهربية تساوى 2.1 أو 2.2 .

δEN تكون هى الفرق في السالبية الكهربية لأى ذرتين أو عنصرين . وكقاعدة عامة يكون نوع الرابطة بين ذرتين رابطة أيونية في حالة أن الفرق في السالبية الكهربية بينهما ( أكبر من أو يساوي 1.7 . وعندما يكون الفرق في السالبية الكهربية ( 0.4 - 1.7 ) فإن الرابطة تعتبر تساهمية قطبية ، وعندما يكون الفرق أقل من 0.4 تعتبر الرابطة تساهمية غير قطبية ، وعندما يكون الفرق مساويا للصفر فإن الرابطة تكون رابطة تساهمية غير قطبية تماما.

The difference in electronegativity between atoms A and B is given by:

$$|\chi _{\rm {A}}-\chi _{\rm {B}}|=({\rm {eV}})^{-1/2}{\sqrt {E_{\rm {d}}({\rm {AB}})-{\frac {E_{\rm {d}}({\rm {AA}})+E_{\rm {d}}({\rm {BB}})}{2}}}}$$

where the dissociation energies, E_d, of the A–B, A–A and B–B bonds are expressed in electronvolts, the factor (eV)^−1⁄₂ being included to ensure a dimensionless result. Hence, the difference in Pauling electronegativity between hydrogen and bromine is 0.73 (dissociation energies: H–Br, 3.79 eV; H–H, 4.52 eV; Br–Br 2.00 eV)

As only differences in electronegativity are defined, it is necessary to choose an arbitrary reference point in order to construct a scale. Hydrogen was chosen as the reference, as it forms covalent bonds with a large variety of elements: its electronegativity was fixed first at 2.1, later revised to 2.20. It is also necessary to decide which of the two elements is the more electronegative (equivalent to choosing one of the two possible signs for the square root). This is usually done using "chemical intuition": in the above example, hydrogen bromide dissolves in water to form H⁺ and Br⁻ ions, so it may be assumed that bromine is more electronegative than hydrogen. However, in principle, since the same electronegativities should be obtained for any two bonding compounds, the data are in fact overdetermined, and the signs are unique once a reference point has been fixed (usually, for H or F).

To calculate Pauling electronegativity for an element, it is necessary to have data on the dissociation energies of at least two types of covalent bonds formed by that element. A. L. Allred updated Pauling's original values in 1961 to take account of the greater availability of thermodynamic data, and it is these "revised Pauling" values of the electronegativity that are most often used.

The essential point of Pauling electronegativity is that there is an underlying, quite accurate, semi-empirical formula for dissociation energies, namely:

$$E_{\rm {d}}({\rm {AB}})={\frac {E_{\rm {d}}({\rm {AA}})+E_{\rm {d}}({\rm {BB}})}{2}}+(\chi _{\rm {A}}-\chi _{\rm {B}})^{2}{\rm {eV}}$$

or sometimes, a more accurate fit

$$E_{\rm {d}}({\rm {AB}})={\sqrt {E_{\rm {d}}({\rm {AA}})E_{\rm {d}}({\rm {BB}})}}+1.3(\chi _{\rm {A}}-\chi _{\rm {B}})^{2}{\rm {eV}}$$

These are approximate equations but they hold with good accuracy. Pauling obtained the first equation by noting that a bond can be approximately represented as a quantum mechanical superposition of a covalent bond and two ionic bond-states. The covalent energy of a bond is approximate, by quantum mechanical calculations, the geometric mean of the two energies of covalent bonds of the same molecules, and there is additional energy that comes from ionic factors, i.e. polar character of the bond.

The geometric mean is approximately equal to the arithmetic mean—which is applied in the first formula above—when the energies are of a similar value, e.g., except for the highly electropositive elements, where there is a larger difference of two dissociation energies; the geometric mean is more accurate and almost always gives positive excess energy, due to ionic bonding. The square root of this excess energy, Pauling notes, is approximately additive, and hence one can introduce the electronegativity. Thus, it is these semi-empirical formulas for bond energy that underlie the concept of Pauling electronegativity.

The formulas are approximate, but this rough approximation is in fact relatively good and gives the right intuition, with the notion of the polarity of the bond and some theoretical grounding in quantum mechanics. The electronegativities are then determined to best fit the data.

In more complex compounds, there is an additional error since electronegativity depends on the molecular environment of an atom. Also, the energy estimate can be only used for single, not for multiple bonds. The enthalpy of formation of a molecule containing only single bonds can subsequently be estimated based on an electronegativity table, and it depends on the constituents and the sum of squares of differences of electronegativities of all pairs of bonded atoms. Such a formula for estimating energy typically has a relative error on the order of 10% but can be used to get a rough qualitative idea and understanding of a molecule.

 كهرسلبية موليكن

The correlation between Mulliken electronegativities (x-axis, in kJ/mol) and Pauling electronegativities (y-axis).

يتم حساب الأرقام في مقياس موليكن بعمل متوسط لجهد التأين والألفة الإلكترونية. وعلى هذا يتم التعبير عن السالبية الكهربية مباشرة بوحدات الطاقة، وعادة ما تكون بالإلكترون فولت. وتم إقتراحها في عام 1934 عن طريق روبرت إس مولكين.

Robert S. Mulliken proposed that the arithmetic mean of the first ionization energy (E_i) and the electron affinity (E_ea) should be a measure of the tendency of an atom to attract electrons:

$$\chi ={\frac {E_{\rm {i}}+E_{\rm {ea}}}{2}}$$

As this definition is not dependent on an arbitrary relative scale, it has also been termed absolute electronegativity, with the units of kilojoules per mole or electronvolts. However, it is more usual to use a linear transformation to transform these absolute values into values that resemble the more familiar Pauling values. For ionization energies and electron affinities in electronvolts,

$$\chi =0.187(E_{\rm {i}}+E_{\rm {ea}})+0.17\,$$

and for energies in kilojoules per mole,

$$\chi =(1.97\times 10^{-3})(E_{\rm {i}}+E_{\rm {ea}})+0.19.$$

The Mulliken electronegativity can only be calculated for an element whose electron affinity is known. Measured values are available for 72 elements, while approximate values have been estimated or calculated for the remaining elements.

The Mulliken electronegativity of an atom is sometimes said to be the negative of the chemical potential. By inserting the energetic definitions of the ionization potential and electron affinity into the Mulliken electronegativity, it is possible to show that the Mulliken chemical potential is a finite difference approximation of the electronic energy with respect to the number of electrons., i.e.,

$$\mu ({\rm {Mulliken)=-\chi ({\rm {Mulliken)={}-{\frac {E_{\rm {i}}+E_{\rm {ea}}}{2}}}}}}$$

 كهرسلبية آلرد-روتشو

The correlation between Allred–Rochow electronegativities (x-axis, in Å⁻²) and Pauling electronegativities (محور y).

A. Louis Allred and Eugene G. Rochow considered that electronegativity should be related to the charge experienced by an electron on the "surface" of an atom: The higher the charge per unit area of atomic surface the greater the tendency of that atom to attract electrons. The effective nuclear charge, Z_eff, experienced by valence electrons can be estimated using Slater's rules, while the surface area of an atom in a molecule can be taken to be proportional to the square of the covalent radius, r_cov. When r_cov is expressed in picometres,

$$\chi =3590{{Z_{\rm {eff}}} \over {r_{\rm {cov}}^{2}}}+0.744$$

 Sanderson electronegativity equalization

The correlation between Sanderson electronegativities (x-axis, arbitrary units) and Pauling electronegativities (y-axis).

R.T. Sanderson has also noted the relationship between Mulliken electronegativity and atomic size, and has proposed a method of calculation based on the reciprocal of the atomic volume. With a knowledge of bond lengths, Sanderson's model allows the estimation of bond energies in a wide range of compounds. Sanderson's model has also been used to calculate molecular geometry, s-electron energy, NMR spin-spin coupling constants and other parameters for organic compounds. This work underlies the concept of electronegativity equalization, which suggests that electrons distribute themselves around a molecule to minimize or to equalize the Mulliken electronegativity. This behavior is analogous to the equalization of chemical potential in macroscopic thermodynamics.

 كهرسلبية آلن

The correlation between Allen electronegativities (x-axis, in kJ/mol) and Pauling electronegativities (y-axis).

Perhaps the simplest definition of electronegativity is that of Leland C. Allen, who has proposed that it is related to the average energy of the valence electrons in a free atom,

$$\chi ={n_{\rm {s}}\varepsilon _{\rm {s}}+n_{\rm {p}}\varepsilon _{\rm {p}} \over n_{\rm {s}}+n_{\rm {p}}}$$

where ε_s,p are the one-electron energies of s- and p-electrons in the free atom and n_s,p are the number of s- and p-electrons in the valence shell.

The one-electron energies can be determined directly from spectroscopic data, and so electronegativities calculated by this method are sometimes referred to as spectroscopic electronegativities. The necessary data are available for almost all elements, and this method allows the estimation of electronegativities for elements that cannot be treated by the other methods, e.g. francium, which has an Allen electronegativity of 0.67. However, it is not clear what should be considered to be valence electrons for the d- and f-block elements, which leads to an ambiguity for their electronegativities calculated by the Allen method.

On this scale, neon has the highest electronegativity of all elements, followed by fluorine, helium, and oxygen.

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 التعالق بين الكهرسلبية والخصائص الأخرى

The variation of the isomer shift (y-axis, in mm/s) of [SnX₆]²⁻ anions, as measured by ¹¹⁹Sn Mössbauer spectroscopy, against the sum of the Pauling electronegativities of the halide substituents (x-axis).

The wide variety of methods of calculation of electronegativities, which all give results that correlate well with one another, is one indication of the number of chemical properties that might be affected by electronegativity. The most obvious application of electronegativities is in the discussion of bond polarity, for which the concept was introduced by Pauling. In general, the greater the difference in electronegativity between two atoms the more polar the bond that will be formed between them, with the atom having the higher electronegativity being at the negative end of the dipole. Pauling proposed an equation to relate the "ionic character" of a bond to the difference in electronegativity of the two atoms, although this has fallen somewhat into disuse.

Several correlations have been shown between infrared stretching frequencies of certain bonds and the electronegativities of the atoms involved: however, this is not surprising as such stretching frequencies depend in part on bond strength, which enters into the calculation of Pauling electronegativities. More convincing are the correlations between electronegativity and chemical shifts in NMR spectroscopy or isomer shifts in Mössbauer spectroscopy (see figure). Both these measurements depend on the s-electron density at the nucleus, and so are a good indication that the different measures of electronegativity really are describing "the ability of an atom in a molecule to attract electrons to itself".

 مناحي في الكهرسلبية

 مناحي دورية

The variation of Pauling electronegativity (y-axis) as one descends the main groups of the periodic table from the second period to the sixth period

In general, electronegativity increases on passing from left to right along a period and decreases on descending a group. Hence, fluorine is the most electronegative of the elements (not counting noble gases), whereas caesium is the least electronegative, at least of those elements for which substantial data is available.

There are some exceptions to this general rule. Gallium and germanium have higher electronegativities than aluminium and silicon, respectively, because of the d-block contraction. Elements of the fourth period immediately after the first row of the transition metals have unusually small atomic radii because the 3d-electrons are not effective at shielding the increased nuclear charge, and smaller atomic size correlates with higher electronegativity (see Allred-Rochow electronegativity and Sanderson electronegativity above). The anomalously high electronegativity of lead, in particular when compared to thallium and bismuth, is an artifact of electronegativity varying with oxidation state: its electronegativity conforms better to trends if it is quoted for the +2 state with a Pauling value of 1.87 instead of the +4 state.

 اتجاه السالبية الكهربية

لكل عنصر كيميائي سالبية كهربية مميزة تتراوح بين صفر - 4 على مقياس باولنج. الفلور هو أعلى العناصر في السالبية الكهربية 3.98, بينما أقل العناصر سالبية كهربية هو الفرانسيوم وله قيمة تبلغ 0.7 وبصفة عامة تقل السالية الكهربية كلما اتجهنا لأسفل في الجدول الدوري وتزيد في الغتجاه العرضي كما موضح بالأسفل. وخلال الدورة فإن اللا فلزات تميل لإمتساب الإلكترونات بينما تميل الفلزات لفقدها وهذا راجع لميل الذرة للوصول إلى التركيب الثماني . وبالنزول خلال الدورة فإن تأثير شحنة النواة يقل على غلاف الطاقة الخارجي . وعلى هذا فإن أكثر العناصر سالبية كهربية هى العناصر الموجودة في أعلى الجدول, وأقلها سالبية كهربية يوجد في أسفل الجدول. وبالتالى فإنه بصفة عامة نصف القطر الذري يقل في الإتجاه العرضي للجدول الدوري, ولكن طاقة التأين تزيد في نفس الإتجاه .

← نصف القطر الذري يتناقص ← طاقة التأين تتزايد ← السالبية الكهربية تتزايد ←
المجموعة	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18
Period
1	H 2.20																	He
2	Li 0.98	Be 1.57											B 2.04	C 2.55	N 3.04	O 3.44	F 3.98	Ne
3	Na 0.93	Mg 1.31											Al 1.61	Si 1.90	P 2.19	S 2.58	Cl 3.16	Ar
4	K 0.82	Ca 1.00	Sc 1.36	Ti 1.54	V 1.63	Cr 1.66	Mn 1.55	Fe 1.83	Co 1.88	Ni 1.91	Cu 1.90	Zn 1.65	Ga 1.81	Ge 2.01	As 2.18	Se 2.55	Br 2.96	Kr 3.00
5	Rb 0.82	Sr 0.95	Y 1.22	Zr 1.33	Nb 1.6	Mo 2.16	Tc 1.9	Ru 2.2	Rh 2.28	Pd 2.20	Ag 1.93	Cd 1.69	In 1.78	Sn 1.96	Sb 2.05	Te 2.1	I 2.66	Xe 2.6
6	Cs 0.79	Ba 0.89	Lu 1.27	Hf 1.3	Ta 1.5	W 2.36	Re 1.9	Os 2.2	Ir 2.20	Pt 2.28	Au 2.54	Hg 2.00	Tl 1.62	Pb 2.33	Bi 2.02	Po 2.0	At 2.2	Rn
7	Fr 0.7	Ra 0.9	Lr	Rf	Db	Sg	Bh	Hs	Mt	Ds	Rg	Uub	Uut	Uuq	Uup	Uuh	Uus	Uuo

الجدول الدوري موضحا عليه السالبية الكهربية بمقياس باولنج

 التفاوت في الكهرسلبية مع رقم الأكسدة

In inorganic chemistry it is common to consider a single value of the electronegativity to be valid for most "normal" situations. While this approach has the advantage of simplicity, it is clear that the electronegativity of an element is not an invariable atomic property and, in particular, increases with the oxidation state of the element.

Allred used the Pauling method to calculate separate electronegativities for different oxidation states of the handful of elements (including tin and lead) for which sufficient data was available. However, for most elements, there are not enough different covalent compounds for which bond dissociation energies are known to make this approach feasible. This is particularly true of the transition elements, where quoted electronegativity values are usually, of necessity, averages over several different oxidation states and where trends in electronegativity are harder to see as a result.

الحمض	الصيغة	حالة أكسدة الكلور	pKa
Hypochlorous acid	HClO	+1	+7.5
Chlorous acid	HClO2	+3	+2.0
Chloric acid	HClO3	+5	–1.0
Perchloric acid	HClO4	+7	–10

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 الكهرإيجابية

الكهرإيجابية Electropositivity هي مقياس لقدرة عنصر على التبرع بإلكترونات، وبذلك يشكـِّل أيونات موجبة؛ ولذلك فهي معاكسة للكهرسلبية.

 انظر أيضاً

• Electronegativities of the elements (data page)
• Chemical polarity

 ببليوجرافيا

• Jolly, William L. (1991). Modern Inorganic Chemistry (2nd ed.). New York: McGraw-Hill. pp. 71–76. ISBN 0-07-112651-1.
• Mullay, J. (1987). "Estimation of atomic and group electronegativities". Structure and Bonding. Structure and Bonding. 66: 1–25. doi:10.1007/BFb0029834. ISBN 3-540-17740-X.

 وصلات خارجية

• WebElements, lists values of electronegativities by a number of different methods of calculation
• Video explaining electronegativity