1.8 Ionic Bonds

An illustration shows a crystal lattice of solid sodium chloride. The crystal lattice of sodium chloride is in the shape of a cuboid. It consists of tightly packed large spheres, denoting chloride ion and small spheres, denoting sodium ions. Both the spheres are shown arranged alternatively on each face of the shape. The caption reads, �Representation of the solid crystal structure of solid sodium chloride. The sodium cation and chlorine anion are held together by electrostatic forces, called ionic bonds, whereby opposite charges attract. As a solid, the ions form a regular array called a crystal lattice.�
FIGURE 1-19 Representation of the solid crystal structure of NaCl(s) The Na+ and Cl– ions are held together by electrostatic forces, called ionic bonds, whereby opposite charges attract. As a solid, the ions form a regular array called a crystal lattice.

When elements in a compound have large enough differences in electronegativity, ionic bonding can occur. Rather than sharing electrons, as in a covalent bond, the more electronegative atom acquires electrons given up by the less electronegative atom, forming oppositely charged ions. The electrostatic attraction between the positively charged cations and the negatively charged anions constitutes the ionic bond. Sodium chloride (NaCl), for example, consists of sodium cations (Na+) and chloride anions (Cl–). Sodium is a metal from the left side of the periodic table, so it has a low electronegativity value (0.93), whereas chlorine is a nonmetal from the right side of the periodic table, so it has a high electronegativity value (3.16).

Most ionic compounds (e.g., NaCl and MgBr2) consist of a metal and a nonmetal, whereas most covalent compounds (e.g., CH4) consist of nonmetals only.

While covalent compounds are generally found as discrete uncharged molecules, the ions in an ionic solid are arranged in a regular array, called a crystal lattice, as shown for NaCl in Figure 1-19. As we will discuss in greater detail in Section 2.7, ionic compounds such as this tend to dissolve in water as their constituent ions.

YOUR TURN 1.7
SHOW ANSWERS

Calculate the difference in electronegativity between the elements in NaCl (an ionic compound) and those in CH4 (a covalent compound). What do you notice?

NaCl ________________ CH4 ________________

For NaCl, which is ionic, the difference is large: 3.16 – 0.93 = 2.23. For CH4, which is covalent, the difference is small: 2.55 – 2.20 = 0.35.

Polyatomic ions, such as the hydroxide (HO–), methoxide (CH3O–), and methylammonium ions shown in Figure 1-20, contain more than one atom. Polyatomic ions usually consist only of nonmetals, and their atoms are held together by covalent bonds.

Figure 1-20 also shows that polyatomic ions can be either anions (HO– and CH3O–) or cations (). Most polyatomic cations possess a nitrogen atom participating in four covalent bonds (another common example is ammonium ion, ). The reasons why are discussed in Section 1.9. Interestingly, the existence of these kinds of polyatomic cations makes possible the formation of ionic compounds composed entirely of nonmetals (e.g., CH3NH3Cl or NH4Br).

Three condensed structural formulae of polyatomic ions in ionic compounds and the covalent bonds that form between them. The first condensed structural formula shows sodium cation with a hydroxide ion bearing a negative charge. The oxygen of the hydroxide ion carries three lone pairs of electrons and is single bonded to a hydrogen atom. The second condensed structural formula shows a potassium cation with a methoxide ion bearing a negative charge. The methoxide ion consists of a central carbon atom, single bonded to three hydrogen atoms, and an oxygen atom carrying three lone pairs of electrons. The third condensed structural formula shows a chloride anion with a methylammonium ion bearing a positive charge. The methylammonium ion consists of a carbon atom, single bonded to a nitrogen atom. Each carbon and nitrogen atom is single bonded to three hydrogen atoms. The sodium and potassium cations consist of the atoms found on the left side of the periodic table. The hydroxide ion, methoxide ion, and methylammonium ion consist of the atoms found on the right side of the periodic table, except for hydrogen atoms. The caption reads, �Polyatomic ions in ionic compounds: Polyatomic ions are usually composed of nonmetal elements from the right-hand side of the periodic table. Those elements have similar electronegativities, allowing covalent bonds to form between them.�
FIGURE 1-20 Polyatomic ions in ionic compounds Polyatomic ions are usually composed of nonmetal elements from the right-hand side of the periodic table. Those elements have similar electronegativities, allowing covalent bonds to form between them.

Solved Problem 1.14

Identify each of the following as either an ionic compound (i.e., one containing ionic bonds) or a covalent compound (i.e., one containing only covalent bonds): (a) NH4CHO2; (b) LiOCH2CH3; (c) CH3CH2CH2OH.

Think
SHOW SECTION

Does the compound contain elements from both the left and right sides of the periodic table? Does the compound contain any recognizable polyatomic cations?

Solve
SHOW SECTION

 (a) Ionic compound: is a common polyatomic cation, leaving as the anion.

 (b) Ionic compound: Li is a metal from the left side of the periodic table, and the remaining elements, which compose the CH3CH2O– anion, are nonmetals from the right side.

 (c) Covalent compound: All elements are nonmetals from the right side of the periodic table, and no recognizable polyatomic ions are present.

problem 1.15 Which of the following are ionic compounds (i.e., ones containing ionic bonds) and which are covalent compounds (i.e., ones containing only covalent bonds)?

Condensed structural formulae of two compounds labeled A and B, and chemical formula of one compound labeled C. The first condensed structure has a hexagon ring with alternating single and double bonds inside. One of the carbon atoms is bonded to an oxygen atom, which is further bonded to a sodium atom. The rest of the five carbon atoms are bonded to a hydrogen atom each. The second condensed structure shows a central carbon atom, single bonded to two methyl groups and double bonded to an oxygen atom. The chemical formula reads, CH3CH2NH3Cl.