MCAT General Chemistry Review
Chapter 3: Bonding and Chemical Interactions
3.2 Ionic Bonds
Ionic bonds form between atoms that have significantly different electronegativities. The atom that loses the electrons becomes a cation, and the atom that gains electrons becomes an anion. The ionic bond is the result of an electrostatic force of attraction between the opposite charges of these ions. Electrons are not shared in an ionic bond. For this electron transfer to occur, the difference in electronegativity must be greater than 1.7 on the Pauling scale.
· MeTals lose electrons to become caTions = posiTive (+) ions
· Nonmetals gain electrons to become aNions = Negative (–) ions
The MCAT won’t expect you to memorize the Pauling scale, but recognize that ionic bonds are generally formed between a metal and a nonmetal. For example, alkali and alkaline earth metals of Groups IA and IIA (Groups 1 and 2, respectively) readily form ionic bonds with the halogens of Group VIIA (Group 17). The atoms of the active metals loosely hold onto their electrons, whereas the halogens are more likely to gain an electron to complete their valence shell. The differences in bonding behavior for these classes of elements, and their differences in electronegativity values (ΔEN), explain the formation of ionic compounds such as cesium chloride (ΔEN = 2.3), potassium iodide (ΔEN = 1.7), and sodium fluoride (ΔEN = 3.1).
Ionic compounds have characteristic physical properties that you should recognize for Test Day. Because of the strength of the electrostatic force between the ionic constituents of the compound, ionic compounds have very high melting and boiling points. For example, the melting point of sodium chloride is 801°C. Many ionic compounds dissolve readily in aqueous and other polar solvent solutions and, in the molten or aqueous state, are good conductors of electricity. In the solid state, the ionic constituents of the compound form a crystalline lattice consisting of repeating positive and negative ions, as shown earlier in Figure 3.3. With this arrangement, the attractive forces between oppositely charged ions are maximized, and the repulsive forces between ions of like charge are minimized.
MCAT Concept Check 3.2:
Before you move on, assess your understanding of the material with these questions.
1. Why do ionic bonds tend to form between metals and nonmetals?
2. Describe five characteristics of ionic compounds.