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Derived copy of Ionic Bonding*

Sylvia K. Quick

Based on Ionic Bonding� by

OpenStax

This work is produced by OpenStax-CNX and licensed under the

Creative Commons Attribution License 4.0�

Abstract

By the end of this section, you will be able to:

• Explain the formation of cations, anions, and ionic compounds• Predict the charge of common metallic and nonmetallic elements, and write their electron con�gu-

rations

As you have learned, ions are atoms or molecules bearing an electrical charge. A cation (a positive ion)forms when a neutral atom loses one or more electrons from its valence shell, and an anion (a negative ion)forms when a neutral atom gains one or more electrons in its valence shell.

Compounds composed of ions are called ionic compounds (or salts), and their constituent ions are heldtogether by ionic bonds: electrostatic forces of attraction between oppositely charged cations and anions.The properties of ionic compounds shed some light on the nature of ionic bonds. Ionic solids exhibit acrystalline structure and tend to be rigid and brittle; they also tend to have high melting and boiling points,which suggests that ionic bonds are very strong. Ionic solids are also poor conductors of electricity forthe same reason�the strength of ionic bonds prevents ions from moving freely in the solid state. Mostionic solids, however, dissolve readily in water. Once dissolved or melted, ionic compounds are excellentconductors of electricity and heat because the ions can move about freely.

Neutral atoms and their associated ions have very di�erent physical and chemical properties. Sodiumatoms form sodium metal, a soft, silvery-white metal that burns vigorously in air and reacts explosivelywith water. Chlorine atoms form chlorine gas, Cl2, a yellow-green gas that is extremely corrosive to mostmetals and very poisonous to animals and plants. The vigorous reaction between the elements sodium andchlorine forms the white, crystalline compound sodium chloride, common table salt, which contains sodiumcations and chloride anions (Figure 1). The compound composed of these ions exhibits properties entirelydi�erent from the properties of the elements sodium and chlorine. Chlorine is poisonous, but sodium chlorideis essential to life; sodium atoms react vigorously with water, but sodium chloride simply dissolves in water.

*Version 1.1: Aug 17, 2016 2:24 am +0000�http://cnx.org/content/m51047/1.8/�http://creativecommons.org/licenses/by/4.0/

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Figure 1: (a) Sodium is a soft metal that must be stored in mineral oil to prevent reaction with air orwater. (b) Chlorine is a pale yellow-green gas. (c) When combined, they form white crystals of sodiumchloride (table salt). (credit a: modi�cation of work by �Jurii�/Wikimedia Commons)

1 The Formation of Ionic Compounds

Binary ionic compounds are composed of just two elements: a metal (which forms the cations) and anonmetal (which forms the anions). For example, NaCl is a binary ionic compound. We can think about theformation of such compounds in terms of the periodic properties of the elements. Many metallic elementshave relatively low ionization potentials and lose electrons easily. These elements lie to the left in a periodor near the bottom of a group on the periodic table. Nonmetal atoms have relatively high electron a�nitiesand thus readily gain electrons lost by metal atoms, thereby �lling their valence shells. Nonmetallic elementsare found in the upper-right corner of the periodic table.

As all substances must be electrically neutral, the total number of positive charges on the cations ofan ionic compound must equal the total number of negative charges on its anions. The formula of anionic compound represents the simplest ratio of the numbers of ions necessary to give identical numbers ofpositive and negative charges. For example, the formula for aluminum oxide, Al2O3, indicates that this ioniccompound contains two aluminum cations, Al3+, for every three oxide anions, O2− [thus, (2 × +3) + (3 ×�2) = 0].

It is important to note, however, that the formula for an ionic compound does not represent the physicalarrangement of its ions. It is incorrect to refer to a sodium chloride (NaCl) �molecule� because there is nota single ionic bond, per se, between any speci�c pair of sodium and chloride ions. The attractive forcesbetween ions are isotropic�the same in all directions�meaning that any particular ion is equally attractedto all of the nearby ions of opposite charge. This results in the ions arranging themselves into a tightlybound, three-dimensional lattice structure. Sodium chloride, for example, consists of a regular arrangementof equal numbers of Na+ cations and Cl� anions (Figure 2).

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Figure 2: The atoms in sodium chloride (common table salt) are arranged to (a) maximize oppositecharges interacting. The smaller spheres represent sodium ions, the larger ones represent chloride ions.In the expanded view (b), the geometry can be seen more clearly. Note that each ion is �bonded� to allof the surrounding ions�six in this case.

The strong electrostatic attraction between Na+ and Cl� ions holds them tightly together in solid NaCl.It requires 769 kJ of energy to dissociate one mole of solid NaCl into separate gaseous Na+ and Cl� ions:

NaCl (s) −→ Na+ (g) + Cl� (g) ∆H = 769 kJ (2)

2 Key Concepts and Summary

Atoms gain or lose electrons to form ions with particularly stable electron con�gurations. The charges ofcations formed by the representative metals may be determined readily because, with few exceptions, theelectronic structures of these ions have either a noble gas con�guration or a completely �lled electron shell.The charges of anions formed by the nonmetals may also be readily determined because these ions formwhen nonmetal atoms gain enough electrons to �ll their valence shells.

3 Chemistry End of Chapter Exercises

Exercise 1 (Solution on p. 5.)

Does a cation gain protons to form a positive charge or does it lose electrons?

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Exercise 2Iron(III) sulfate [Fe2(SO4)3] is composed of Fe3+ and SO4

2− ions. Explain why a sample of iron(III)sulfate is uncharged.

Exercise 3 (Solution on p. 5.)

Which of the following atoms would be expected to form negative ions in binary ionic compoundsand which would be expected to form positive ions: P, I, Mg, Cl, In, Cs, O, Pb, Co?

Exercise 4Which of the following atoms would be expected to form negative ions in binary ionic compoundsand which would be expected to form positive ions: Br, Ca, Na, N, F, Al, Sn, S, Cd?

Exercise 5 (Solution on p. 5.)

Predict the charge on the monatomic ions formed from the following atoms in binary ionic com-pounds:

(a) P(b) Mg(c) Al(d) O(e) Cl(f) Cs

Exercise 6Predict the charge on the monatomic ions formed from the following atoms in binary ionic com-pounds:

(a) I(b) Sr(c) K(d) N(e) S(f) In

Exercise 7From the labels of several commercial products, prepare a list of six ionic compounds in theproducts. For each compound, write the formula. (You may need to look up some formulas in asuitable reference.)

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Solutions to Exercises in this Module

Solution to Exercise (p. 3)The protons in the nucleus do not change during normal chemical reactions. Only the outer electrons move.Positive charges form when electrons are lost.Solution to Exercise (p. 4)P, I, Cl, and O would form anions because they are nonmetals. Mg, In, Cs, Pb, and Co would form cationsbecause they are metals.Solution to Exercise (p. 4)(a) P3�; (b) Mg2+; (c) Al3+; (d) O2�; (e) Cl�; (f) Cs+

Glossary

De�nition 2: inert pair e�ecttendency of heavy atoms to form ions in which their valence s electrons are not lost

De�nition 2: ionic bondstrong electrostatic force of attraction between cations and anions in an ionic compound

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