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What Is Electronegativity and How Does It Work?

ThoughtCo/Todd Helmenstine

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Electronegativity is the property of an atom which increases with its tendency to attract the electrons of a bond. If two bonded atoms have the same electronegativity values as each other, they share electrons equally in a covalent bond. Usually, the electrons in a chemical bond are more attracted to one atom (the more electronegative one) than to the other. This results in a polar covalent bond. If the electronegativity values are very different, the electrons aren't shared at all. One atom essentially takes the bond electrons from the other atom, forming an ionic bond.

Key Takeaways: Electronegativity

• Electronegativity is an atom's tendency to attract electrons to itself in a chemical bond.
• The most electronegative element is fluorine. The least electronegative or most electropositive element is francium.
• The greater the difference between atom electronegativity values, the more polar the chemical bond formed between them.

Avogadro and other chemists studied electronegativity before it was formally named by Jöns Jacob Berzelius in 1811. In 1932, Linus Pauling proposed an electronegativity scale based on bond energies. Electronegativity values on the Pauling scale are dimensionless numbers that run from about 0.7 to 3.98. The Pauling scale values are relative to the electronegativity of hydrogen (2.20). While the Pauling scale is most often used, other scales include the Mulliken scale, Allred-Rochow scale, Allen scale, and Sanderson scale.

Electronegativity is a property of an atom within a molecule, rather than an inherent property of an atom by itself. Thus, electronegativity actually varies depending on an atom's environment. However, most of the time an atom displays similar behavior in different situations. Factors that affect electronegativity include the nuclear charge and the number and location of electrons in an atom.

Electronegativity Example

The chlorine atom has a higher electronegativity than the hydrogen atom, so the bonding electrons will be closer to the Cl than to the H in the HCl molecule.

In the O 2 molecule, both atoms have the same electronegativity. The electrons in the covalent bond are shared equally between the two oxygen atoms.

Most and Least Electronegative Elements

The most electronegative element on the periodic table is fluorine (3.98). The least electronegative element is cesium (0.79). The opposite of electronegativity is electropositivity, so you could simply say cesium is the most electropositive element. Note that older texts list both francium and cesium as least electronegative at 0.7, but the value for cesium was experimentally revised to the 0.79 value. There is no experimental data for francium, but its ionization energy is higher than that of cesium, so it is expected that francium is slightly more electronegative.

Electronegativity as a Periodic Table Trend

Like electron affinity, atomic/ionic radius, and ionization energy, electronegativity shows a definite trend on the periodic table .

• Electronegativity generally increases moving from left to right across a period. The noble gases tend to be exceptions to this trend.
• Electronegativity generally decreases moving down a periodic table group. This correlates with the increased distance between the nucleus and the valence electron.

Electronegativity and ionization energy follow the same periodic table trend. Elements that have low ionization energies tend to have low electronegativities. The nuclei of these atoms don't exert a strong pull on electrons . Similarly, elements that have high ionization energies tend to have high electronegativity values. The atomic nucleus exerts a strong pull on electrons.

Jensen, William B. "Electronegativity from Avogadro to Pauling: Part 1: Origins of the Electronegativity Concept." 1996, 73, 1. 11, J. Chem. Educ., ACS Publications, January 1, 1996.

Greenwood, N. N. "Chemistry of the Elements." A. Earnshaw, (1984). 2nd Edition, Butterworth-Heinemann, December 9, 1997.

Pauling, Linus. "The Nature of the Chemical Bond. IV. The Energy of Single Bonds and the Relative Electronegativity of Atoms". 1932, 54, 9, 3570-3582, J. Am. Chem. Soc., ACS Publications, September 1, 1932.

Pauling, Linus. "The Nature of the Chemical Bond and the Structure of Molecules and Crystals: An Introduction to Mode." 3rd Edition, Cornell University Press, January 31, 1960.

• Examples of Polar and Nonpolar Molecules
• How to Use a Periodic Table of Elements
• Periodic Table Definition in Chemistry
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• What Is the Most Electronegative Element?
• Learn Which Element Has the Lowest Electronegativity Value
• The Periodic Properties of the Elements
• Most Reactive Metal on the Periodic Table
• Cool Chemical Element Facts
• Why Do Atoms Create Chemical Bonds?
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electronegativity , in chemistry , the ability of an atom to attract to itself an electron pair shared with another atom in a chemical bond.

The commonly used measure of the electronegativities of chemical elements is the electronegativity scale derived by Linus Pauling in 1932. In it the elements are tabulated in decreasing order of electronegativity, fluorine being the most electronegative and cesium the least. The scale was derived from a comparison of the energies associated with chemical bonds between various combinations of atoms. A scale very similar to Pauling’s values has been obtained by measurements of atomic ionization potentials and electron affinities .

Elements differing greatly in electronegativity tend to form ionic compounds , composed of positively and negatively charged units called ions; those differing moderately in electronegativity form polar, covalent compounds, in which atoms are held together by chemical bonds but which show some degree of ionization, while those elements with approximately equal electronegativities form nonpolar compounds, which show little charge separation.

Electronegativity—a perspective

• Original Paper
• Published: 23 July 2018
• Volume 24 , article number  214 , ( 2018 )

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• Peter Politzer 1 &
• Jane S. Murray 1  

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Electronegativity is a very useful concept but it is not a physical observable; it cannot be determined experimentally. Most practicing chemists view it as the electron-attracting power of an atom in a molecule. Various formulations of electronegativity have been proposed on this basis, and predictions made using different formulations generally agree reasonably well with each other and with chemical experience. A quite different approach, loosely linked to density functional theory, is based on a ground-state free atom or molecule, and equates electronegativity to the negative of an electronic chemical potential. A problem that is encountered with this approach is the differentiation of a noncontinuous function. We show that this approach leads to some results that are not chemically valid. A formulation of atomic electronegativity that does prove to be effective is to express it as the average local ionization energy on an outer contour of the atom’s electronic density.

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Politzer, P., Murray, J.S. Electronegativity—a perspective. J Mol Model 24 , 214 (2018). https://doi.org/10.1007/s00894-018-3740-6

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• Published: 26 April 2017

Electronegativity determination of individual surface atoms by atomic force microscopy

• Jo Onoda 1 , 2 ,
• Martin Ondráček 3 ,
• Pavel Jelínek 3 , 4 &
• Yoshiaki Sugimoto 1 , 2  

Nature Communications volume  8 , Article number:  15155 ( 2017 ) Cite this article

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Atomic force microscopy

• Characterization and analytical techniques
• Scanning probe microscopy

Electronegativity is a fundamental concept in chemistry. Despite its importance, the experimental determination has been limited only to ensemble-averaged techniques. Here, we report a methodology to evaluate the electronegativity of individual surface atoms by atomic force microscopy. By measuring bond energies on the surface atoms using different tips, we find characteristic linear relations between the bond energies of different chemical species. We show that the linear relation can be rationalized by Pauling’s equation for polar covalent bonds. This opens the possibility to characterize the electronegativity of individual surface atoms. Moreover, we demonstrate that the method is sensitive to variation of the electronegativity of given atomic species on a surface due to different chemical environments. Our findings open up ways of analysing surface chemical reactivity at the atomic scale.

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Introduction.

Electronegativity, an important theoretical concept in chemistry, was originally defined by Linus Pauling as ‘the power of an atom in a molecule to attract electrons to itself’ 1 , 2 . Since that time, various classical scales of electronegativity have been suggested 3 , 4 , 5 , 6 , including the rigorous modern formalism of absolute electronegativity developed along with density functional theory (DFT) 7 . In contrast, conventional experimental means for measuring electronegativity have been limited to thermochemical techniques, that is, the measurement of ensemble-averaged bond energies 8 . On the other hand, atomic force microscopy (AFM) was able to achieve atomic resolution on both semiconductor 9 and insulator 10 surfaces. It has also found numerous applications in the field of chemistry: for example, quantitative measurement of short-range chemical forces 11 , chemical identification of individual surface atoms 12 , visualization of the internal structures of organic molecules 13 , 14 , 15 , 16 and metal clusters 17 , discrimination of the Pauling bond order 18 , 19 and tracking of surface chemical reactions 20 , 21 .

Here, we extend this already impressive suite of applications by demonstrating the use of AFM in characterizing the electronegativity of individual surface atoms. Pauling’s electronegativity values for individual single atoms on surfaces can be estimated using site-specific energy spectroscopy acquired with a variety of AFM tips. Namely, we show that the binding energies for individual surface atoms observed using different tips can provide an ensemble of data that can be used later to determine the electronegativity of different chemical elements or chemical groups on a surface ( Fig. 1a ). Our experimental findings are supported by theoretical analysis based on DFT calculations.

( a ) Schematic illustration of AFM energy spectroscopy with the polar covalent bond of Si–O. ( b ) Δ f curves measured on Si and O adatoms on the Si(111)-(7 × 7) surface. The background (BG) Δ f ( z ) curve is also included for subtraction of the long-range component (Methods). ( c ) E ( z ) curves converted from short-range components in the Δ f ( z ) curves in b . Bond energies are defined at the minima of E ( z ) curves. The acquisition parameters are f 0 =152.913 kHz, the oscillation amplitude ( A )=146 Å, the spring constant ( k )=28.2 N m −1 and sample bias ( V S )=+40 mV.

Scatter plots of the bond energies

Scatter plots of the bond energies of Ge ( a ), Sn ( b ), Al ( c ), O ( d ) adatoms, SiO 2 ( e ) and SiNO ( f ) complexes obtained experimentally. The bond energies were measured above the bright spots in AFM images as shown in the insets. The individual error bars are estimated as 10% of the corresponding bond energies based on uncertainties in measurements of A and k . The scatter plots were fitted using weighted orthogonal distance regression.

Interpretation of the linear relations by Pauling’s equation

We rationalized the linear relation between the bond energy of tip apexes and different surface atoms by employing Pauling’s equation for the bond energy E A–B of heterogeneous polar bonds between atoms A and B 1 , 2 :

Chemical identification by the slopes

Electronegativity determination by the intercepts.

( a ) A one-dimensional plot showing the variation of the Hartree potential above different adatoms such as Si, Ge, Al, O or N. The zero point of the Hartree potential is set to the Fermi level, and the z -distance is aligned to the height of an inspected adatom ( z X ) on the Si(111)-(7 × 7) surface. The calculated electron density isosurfaces (0.02 e Å −3 in yellow) of the highest occupied frontiers orbital on the Si adatom ( b ), SiO 2 ( c ) around the Fermi level and SiNO ( d ) located 1 eV below the Fermi level. Cut-plane of the electron density isosurface is coloured from red (0.13 e Å −3 ) to blue (0.02 e Å −3 ). Plotted two-dimensional plane of the Hartree potential projected onto the Si adatom ( e ), SiO 2 ( f ) and SiNO ( g ) complex (range from 0 (blue) to 5 (red) eV).

Evaluation of group electronegativity

In general, the electronegativity of surface atoms (here, Si adatoms) can be modified by their surrounding environments: for example, structural and chemical rearrangement of local structures, charge transfer from the neighbouring atoms and re-hybridization of orbitals. Previously, this information has not been experimentally accessible. From this perspective, the present method provides a unique opportunity to obtain electronegativity values for specific surface atoms in different local chemical structures. To demonstrate such sensitivity towards group electronegativity, we carried out measurements on a ‘SiO 2 ’ structure ( Fig. 2e ), where two O atoms are inserted into the back bonds of an Si adatom 22 , and on a ‘SiNO’ structure ( Fig. 2f ) created from dissociated O and N atoms ( Supplementary Methods 1 ). Local atomic arrangements obtained from the total-energy DFT calculations of the SiO 2 and SiNO structures are presented in Fig. 3c,d , respectively.

In conclusion, we have presented here a method that, for the first time to our knowledge, allows experimental characterization of the electronegativity of individual surface atoms by means of AFM. We have found that the scatter plots of the bond energies can be interpreted based on Pauling’s equation for polar covalent bonds. This allowed us to disentangle the covalent and ionic bond energies in polar covalent bonds, and hence to estimate the electronegativity of the individual elements. In addition, this method facilitates analysis of the group electronegativity of atoms belonging to locally formed nanostructures. We expect that the present method can be applied to other interesting systems such as transition metal oxides used in catalysis by terminating AFM tips with known atoms on the surface. Furthermore, we believe that the method can not only characterize the specific electronegativity of individual atoms, defects, adsorbates and dopant impurities on surfaces but may also open up new ways of analysing surface chemical reactivity in terms of surface electrophilicity/nucleophilicity against reactants 29 , and surface chemical softness/hardness with regard to the hard–soft acid–base principle 26 , 30 .

All experiments were carried out using a custom-built ultrahigh-vacuum AFM system at room temperature with a typical base pressure of 5 × 10 −9  Pa. An optical interferometer was equipped to detect the cantilever deflection. Frequency-modulation technique was used for obtaining Δ f . We used commercial Si cantilevers, whose tip apexes were cleaned by Ar ion sputtering to remove the native oxidation layers. The cantilever was oscillated with sufficiently large amplitude ( A ∼ 100 Å) for stable AFM operation. We chose the Si(111)-(7 × 7) surface as playground for the purpose of the present experiments since the surface had many Si adatoms on which atoms at tip apex were replaced by mild contact. To compensate a contact potential difference between tip and sample, we properly applied V S to the sample while keeping the tip bias at ground.

Site-specific energy spectroscopy

When we performed site-specific energy spectroscopy, thermal drift was compensated by feed-forward technique 31 and the precision of lateral position of the point spectroscopy became better than ±0.1 Å, both of which were realized by our atom-tracking implementation 32 . After recording several Δ f ( z ) curves on an adatom of interest, we averaged them to a single Δ f ( z ) curve for converting to E ( z ). Note that energy dissipation during the spectroscopy was negligible. For eliciting accurate bond energies exerted on the foremost tip atom and surface adatoms, we subtracted the background Δ f ( z ) curve ( Fig. 1b ; Supplementary Methods 1 ), which is obtained by analytically fitting a Δ f ( z ) curve measured on a corner hole site 11 , 12 , from Δ f ( z ) curves on the individual target and reference adatom sites. Finally, the Δ f ( z ) curves only containing the short-range contribution were converted to E ( z ) using the inversion procedure suggested by Sader and Jarvis 33 .

Density functional theory calculations

All DFT calculations have been done using the Vienna Ab-Initio Simulation Package (VASP 4.6) 34 , 35 , which is a pseudopotential plane-wave code. The basic versions of ultrasoft Vanderbilt pseudopotentials 36 , 37 supplied with VASP were used for all chemical elements involved (Si, H, O, N, Al and Ge) apart from Sn, for which the Sn d potential that includes d -electrons as valence electrons was adopted. The PW91 implementation of the generalized gradient approximation 38 was chosen to describe the exchange-correlation functional. The size of the plane-wave basis was determined by the cutoff energy of 300 or 396 eV. Only the central (Gamma) point of the first Brillouin zone was considered for the Bloch-wave solutions.

Data availability

The data that support the findings of this study are available from the corresponding author on reasonable request.

Additional information

How to cite this article: Onoda, J. et al . Electronegativity determination of individual surface atoms by atomic force microscopy. Nat. Commun. 8, 15155 doi: 10.1038/ncomms15155 (2017).

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Acknowledgements

This work was supported by Grants-in-Aid for JSPS Fellows (14J00689) and for Scientific Research (15H03566, 16H00933, 16H00959, 16K13680, 16K17482) from the Ministry of Education, Culture, Sports, Science, and Technology of Japan (MEXT). We also acknowledge financial support from the Asahi Glass Foundation, the Mitsubishi Foundation, the Yamada Science Foundation, Ube foundation, and the Precise Measurement Technology Promotion Foundation. P.J. and M.O. acknowledge financial support of GACR (Czech Science Foundation) under Project no. 14-16963J and Czech Academy of Sciences through Praemium Academiae award.

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Jo Onoda & Yoshiaki Sugimoto

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Martin Ondráček & Pavel Jelínek

Department of Physical Chemistry, Regional Centre of Advanced Technologies and Materials, Palacky University, Šlechtitelů 27, Olomouc, 78371, Czech Republic

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Contributions

J.O. and Y.S. conceived and designed the experiments. J.O. and Y.S. performed the experiments. M.O. and P.J. performed the theoretical analysis. J.O. and P.J. wrote the paper. All authors discussed the results and commented on the manuscript.

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Onoda, J., Ondráček, M., Jelínek, P. et al. Electronegativity determination of individual surface atoms by atomic force microscopy. Nat Commun 8 , 15155 (2017). https://doi.org/10.1038/ncomms15155

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Received : 14 November 2016

Accepted : 27 February 2017

Published : 26 April 2017

DOI : https://doi.org/10.1038/ncomms15155

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