Electronegativity

Type of physical science: Chemistry

Field of study: Chemistry of the elements

Electronegativity, the ability of an atomic nucleus to attract electrons in a chemical bond, is fundamentally important to all aspects of chemistry, explaining the basis for formation of chemical bonds, the nature of these bonds, and the ways in which different elements interact chemically.

Overview

Electronegativity is defined as the relative ability of the atomic nucleus of a given element to attract valence electrons from other elements so as to produce chemical bonds. This concept is essential to understanding all aspects of chemical bond formation, to defining the various types of chemical bonds observed in nature, and to explaining the ways in which the elements interact to undergo chemical reactions. The conceptualization of electronegativity is attributed to Linus Pauling, whose overall work on chemical bonding won for him the Nobel Prize in Chemistry in 1954.

The electronegativity values of the elements are unitless numbers from 0.8 to 4.1, as shown in the table for some representative elements in the main groups of the periodic table of the elements; they were obtained by Pauling. The elements chosen for illustrative purposes in the table below span the entire range of electronegativity values of the elements that exist in nature.

These electronegativity values help chemists to identify both how the various elements react with one another and what the nature of the chemical bonds produced by each given chemical reaction may be.

The table is arranged so that its individual lines, read vertically, represent the seven periods of the periodic table of the elements. The electronegativity values are given as numbers in parentheses. Several generalizations may be drawn from the electronegativity values of the elements shown. These generalizations hold for all the elements that exist, including those not shown. Flourine, whose electronegativity value is 4.1, at the upper right-hand corner of the table, is the most electronegative element in nature. The electronegativity values of the elements increase as one moves from group IA to group VIIA within each line of the table, which represents moving from left to right in any period of the periodic table of the elements. The electronegativities of the elements in any element family decrease from the top of the table to its bottom; this shows that the electronegativities of the elements in each group of elements decrease as one moves from period 1 to period 7. Francium, the element at the bottom-left corner of the table (and of the periodic table), is viewed as being the least electronegative of all the elements.

Examining the entire periodic table shows that the noble gases (group O, not shown in the table) are given electronegativity values of zero, to indicate that they rarely participate in chemical reactions.

The difference between the electronegativities of elements that participate in a chemical bond determines the nature of the bond formed, that is, whether the bond is ionic, covalent, or polar. For example, it is because of the very large differences in the elecronegativities of the elements in groups IA or IIA and those in groups VIA or VIIA that chemical reactions between them yield ionic or very polar bonds. At the other extreme of chemical bond formation, the identical electronegativities of the atoms of the same element allow them to form nonpolar covalent bonds.

The electronegativities of the elements--and the differences between the electronegativities of interactive elements--are quite important to virtually every aspect of chemistry, including the ways in which elements undergo chemical reactions (for example, in redox reactions) and many other chemical phenomena.

Applications

Two applications of electronegativity will be emphasized here: first, that electronegativity differences between any two elements that react chemically determine the nature of the chemical bond produced; and second, the involvement of electronegativity in redox reactions. In most cases, chemical bond formation leads to unequal sharing of electrons. This results from the fact that the participant elements usually possess different electronegativity values. In order to indicate such inequality in electron sharing, the term "polarity" (the extent of this inequity) was developed. Bonds that form when atoms of the same polarity combine are called nonpolar covalent bonds (nonpolar bonds). Furthermore, all bonds in which unequal electron sharing occurs are called polar covalent bonds (polar bonds), and the reaction products produced when they form possess partial positive and partial negative charges, making them dipolar entities. Finally, those chemical bonds produced in cases in which the electrons from one participant atom are transferred completely to another participant atom are called ionic bonds.

When ionic bonds are produced in chemical reactions, the participant atoms become positively charged cations and negatively charged anions, respectively (charged forms of the atoms).

Chemical bonds are nonpolar when the electronegativity difference between participant atoms is zero, polar when the electronegativity difference between the participant atoms is greater than zero and below 1.7, and ionic when the electronegativity differences between the elements involved are about 1.7 or more. Therefore, the understanding of the electronegativities of elements (from the table) is valuable in explaining and predicting the properties of the chemical compounds produced when different elements interact chemically. Furthermore, at this point it may become clear that ionic bonds are actually extremely polar bonds.

Understanding of electronegativities points out that the statement that reaction between between elements in groups IA and VIIA produces ionic compounds is true because the compounds are produced from elements whose electronegativity differences are very large. For example, reaction between lithium (electronegativity 1.0) and fluorine (electronegativity 4.1) produces the very ionic compound lithium fluoride because the electronegativity difference involved is 3.1. Similarly, a chemical bond produced between sodium and chlorine (electronegativities 1.0 and 2.9, respectively) also produces an ionic chemical (electronegativity difference is 1.9), sodium chloride, or table salt. Both sodium chloride and lithium fluoride exist as substances in which the elements produced have become ions (again, charged forms of atoms).

By similar reasoning, a chemical bond produced in a chemical reaction between carbon (electronegativity 2.5) and nitrogen (electronegativity 3.0) will be only weakly polar (electronegativity difference is 0.5). In contrast, a carbon-oxygen bond formed between elements with electronegativity values of 2.5 and 3.5, respectively (electronegativity difference is 1), is more polar, but still not ionic. Finally, all chemical bonds produced by the chemical interaction of two atoms of the same element (for example, two hydrogen atoms forming a diatomic hydrogen molecule) will be nonpolar, because the electronegativity difference in such cases is zero.

Some implicit properties of various kinds of chemicals that result from the types of chemical bonds formed include their physical states at room temperature. Here, for example, chemicals produced by ionic bond formation are usually solids (for example, table salt, sodium chloride), as a result of the very strong attractions between their ions (charged forms of atoms).

Chemicals produced by atomic interactions that yield only polar bonds are usually solids or liquids at room temperature, depending upon the polarities of their constituent bonds and the resultant interactions between them (stronger interactions between the particles of chemicals are required for maintenance of the solid state under given conditions than for maintenance of the liquid state). Finally, chemicals joined together only by nonpolar bonds are usually gases at room temperature, because the interactive forces between them are too low to produce the liquid state.

Another application of electronegativity has to do with redox reactions. Redox reactions are defined as chemical reactions in which one participant chemical is oxidized (loses electrons) and another participant is reduced (gains electrons). Redox reactions are therefore chemical reactions in which electrons flow from one substance to another. For this reason, they are important to the aspect of chemistry called electrochemistry, and they participate in a wide variety of chemical processes including production of electricity, processing of metal ores, and corrosion.

Redox reactions can occur because the different elements possess varying tendencies to be oxidized and reduced. These tendencies are related to their electronegativities. Simplistically stated, the higher the electronegativity value possessed by an element, the greater its tendency to be reduced. Conversely, the lower the electronegativity value possessed by an element, the higher its tendency to be oxidized. Therefore, the metals (and all the other elements) can be placed into a display called a reactivity series, which allows prediction of their abilities to be oxidized or reduced. This series is based on their relative electronegativities and has applications to metal recovery from ores, to electroplating, and to many other societally valuable redox processes.

Context

In the twentieth century, it has become clear that productive reactions between elements yield chemical compounds via interactions of valence electrons. Use of the periodic table and combining characteristics of elements based on their valence electron content allow chemists to predict the numbers of atoms of each element found in element combinations that yield chemical compounds. Among the seminal events here was the 1916 proposal by Gilbert Newton Lewis that interaction between elements to form compounds resulted from sharing electrons.

Lewis was soon shown to be correct, but a badly needed factor missing from the armamentarium of chemical conceptualization of the time was an explanation for the reason that the formation of some chemical compounds was accomplished by unequal sharing of electrons, while equal sharing occurred in other cases: that is, the reason that some element combinations result in ionic bond formation while others lead to covalent bonds, and still others produce nonpolar compounds.

A number of concepts developed in order to facilitate explanation of these phenomena included identification of the sizes of most atoms and ions, and of the amounts of energy involved in making or breaking specific chemical bonds. This information was incorporated into thought about the nature of the forces that produce different types of chemical bonds.

Then, in the late 1920's, Pauling proposed that the basis for the differences in the types of chemical bonds formed between elements was the fact that each element possessed a characteristic ability (electronegativity) to attract electrons in bond formation. This ability, Pauling pointed out, was related to properties of atoms already described by Lewis and others, and he used such information to calculate electronegativity values for the elements.

With Pauling's electronegativity values in hand, chemists were able to clarify the relationships between elements that combined to produce compounds, and to predict with more accuracy the types of chemical compounds that would be produced by specific elemental combinations. Although predictive ability is imperfect, good estimates of the types of chemicals produced in many elemental combinations have become possible.

This information has contributed to the design of specific chemicals to do many desired jobs. It has also been valuable to electrochemistry; understanding the relative electronegativities of elements in the reactivity series of the metals has contributed heavily to the understanding of corrosion, the design of batteries, the production of electronic circuits, and the process of electroplating. Electrochemical applications have been especially germane to the design and development of computer chips, the electronic components hailed as one of the greatest achievements of the twentieth century.

It is expected that future accomplishments in electronics, electrochemistry, other areas of chemistry, and aspects of modern biology will be affected by further development of the conceptualization of electronegativity.

Principal terms

CHEMICAL REACTION: any process in which one or several new substances (reaction products) are produced as a result of chemical changes in chemical starting materials (reactants)

COVALENT BOND: any chemical bond that is produced by the sharing of one or more valence electron pairs between participant atoms; electron sharing may be equal or unequal

ELECTRON: the type of subatomic particle that possesses an electrical charge of -1 and a mass that is 1/1,836 of the mass of a proton

IONIC BOND: any chemical bond that results from the transfer of one or more electrons from an atom or group of atoms to another atom or group of atoms

NONPOLAR BOND: any covalent bond wherein equal sharing of valence electrons by participant atoms occurs

PERIOD: one of the seven horizontal rows of elements in the periodic table of the elements

PERIODIC TABLE: a display table of the known elements arranged in order of their atomic numbers; it organizes them into families and periods, simplifying the systematic study of chemistry

POLAR BOND: a covalent bond in which valence electron sharing between bonded atoms is unequal

REACTIVITY SERIES: a display in which the metals are listed in order of their relative abilities to be oxidized or reduced, useful in understanding the way in which metals interact in redox reactions

REDOX REACTION: any chemical reaction in which one reactant is oxidized (loses valence electrons) and another one is reduced (gains valence electrons)

Bibliography

Hill, John W. CHEMISTRY FOR CHANGING TIMES. 5th ed. New York: Macmillan, 1988. This chatty, liberal arts chemistry textbook is particularly useful for basic concepts related to electronegativity, bond formation, and related concepts. These topics are dealt with in a pleasant, disarming fashion that will educate the beginning reader in the concepts covered. A good beginning for development of a factual base.

Holum, John R. FUNDAMENTALS OF GENERAL, ORGANIC, AND BIOLOGICAL CHEMISTRY. New York: John Wiley & Sons, 1982. This clear chemistry text, aimed at college science majors, does a nice job of explaining the concept of electronegativity and its role in chemical bond formation, redox reactions, and other relevant aspects of chemistry. Many other fundamental concepts pertinent to this article are developed throughout the book, and it is a good source for readers wanting some technical information.

Kieffer, William F. CHEMISTRY TODAY. San Francisco: Canfield Press, 1976. This simple, introductory chemistry text, aimed at the nonscientist, covers many issues of use. Particularly valuable for coverage of oxidation, reduction, and electrochemistry as related to the ability of elements to attract electrons, and to the use of metals (chapter 15).

Pauling, Linus. THE CHEMICAL BOND: A BRIEF INTRODUCTION TO MODERN STRUCTURAL CHEMISTRY. Ithaca, N.Y.: Cornell University Press, 1967. This book has been shortened and simplified for student use. Portions of chapter 3 concern themselves with electronegativity in relatively simple terms, and several useful exercises are included in the chapter.

Pauling, Linus. THE NATURE OF THE CHEMICAL BOND AND THE STRUCTURE OF MOLECULES AND CRYSTALS. Ithaca, N.Y.: Cornell University Press, 1948. This classic text covers many aspects of chemical bonding. Chapter 2 contains valuable information on electronegativity, including the basis for the electronegativity scale of the elements, aspects of matter that allow the most exact expression of elemental electronegativity, and the effect of electronegativity on the nature of chemical bonds. Most useful to those with chemical expertise; it is well worth examining.

Radel, Stanley R., and Marjorie H. Navidi. CHEMISTRY. Saint Paul, Minn.: West, 1990. This general chemistry textbook for college science majors does a good job covering topics germane to understanding electronegativity and its impact on chemical bonding. Contains good sections on the predictive value of electronegativities (chapter 9) and on electrochemistry (chapter 11).

Speakman, J. C. A VALENCY PRIMER. London: Edward Arnold, 1968. This compact book covers much atomic and molecular theory with relatively little mathematics. Topics covered include the molecule and valency, the atom, measurement of molecular properties, and electronegativity. The book is aimed at giving the reader an elementary basis for understanding concepts involved in interactions of atoms to form molecules and the properties of molecules.

Representative elements and their electronegativity values

Chemical Formulas and Combinations

The Periodic Table and the Atomic Shell Model