The Group 7A Elements

Because of their high reactivities, the halogens are not found as free elements in nature. Instead, they are found as halide ions (X ) in various minerals and in seawater (see Table 18.18). 

Although astatine is a member of Group 7A, its chemistry is of no practical importance because all its known isotopes are radioactive. The longest-lived isotope, °At, has a half-life of only 8.3 hours. 

The halogens, particularly fluorine, have very high electronegativity values (see Table 18.17). They tend to form polar covalent bonds with other non-metals and ionic bonds with metals in their lower oxidation states. When a metal ion is in a higher oxidation state, such as +3 or +4, the metal-halogen bonds are polar and covalent. For example, TiCU and SnCU are both covalent compounds that are liquids under normal conditions. 

This reaction occurs with explosive vigor when fluorine and hydrogen are mixed. On the other hand, hydrogen and chlorine can coexist with little apparent reaction for relatively long periods in the dark. However, ultraviolet light causes an explosively fast reaction, and this is the basis of a popular lecture demonstration, the hydrogen-chlorine cannon. Bromine and iodine also react with hydrogen, but more slowly. 

Samples of chlorine gas, liquid bromine, and solid iodine. 

The halogens, particularly fluorine, have very high electronegativity values (Table 19.6). They tend to form polar covalent bonds with other non-metals and ionic bonds with metals in their lower oxidation states. When a 

TABLE 19.6 Trends in Selected Physical Properties of the Group 7A Elements 

A candle burning in an atmosphere of Cl2(g). The exothermic reaction, which involves breaking C—C and C—H bonds in the wax and forming C—Cl bonds in their places, produces enough heat to make the gases in the region incandescent (a flame results). 

Unless otherwise noted, all arton this page Is Cengage Learning2014. 

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Figure 5.1. The graph shows a periodic rise-and-fall pattern. Beginning on the left with atomic number 1 (hydrogen), the size of the atoms increases to a maximum at atomic number 3 (lithium), then decreases to a minimum, then increases again to a maximum at atomic number 11 (sodium), then decreases, and so on. It turns out that all the maxima occur for atoms of group 1A elements—Li (atomic number, Z = 3), Na (Z = 11), K (Z = 19), Rb (Z = 37), Cs (Z = 55), and Fr (Z = 87)—and that the minima occur for atoms of the group 7A elements.

FIGURE 5.1 Agraph of atomic radius in picometers (pm) versus atomic number shows a rise-and-fall pattern of periodicity. The maxima occur for atoms of group 1A elements (Li, Na, K, Rb, Cs, Fr) the minima occur for atoms of the group 7A elements. Accurate data are not available for the group 8A elements. 

Just as atoms shrink when converted to cations by loss of an electron, they expand when converted to anions by gain of an electron. As shown in Figure 6.2 for the group 7A elements (halogens), the expansion is dramatic. Chlorine, for example, nearly doubles in size, from 99 pm for the neutral atom to 184 pm for the chloride anion. 

The relatively large change in size in going from the first to the second member of a group also has important consequences for the Group 7A elements. For example, fluorine has a smaller electron affinity than chlorine. This violation of the expected trend can be attributed to the fact that the small size of the fluorine 2p orbitals causes unusually large electron-electron repulsions. The relative weakness of the bond in the F2 molecule can be explained in terms of the repulsions among the lone pairs, shown in the Lewis structure ... 

TABLE 19.7 Some Physical Properties, Sources, and Methods of Preparation for the Group 7A Elements... 

The elements in group 7A are named for their ability to form compounds with almost all metals. Because these compounds are called salts, the group 7A elements are called salt formers, or halogens. You are famifiar with one salt, sodium chloride, which is known as table salt. The halogens differ in their physical properties, as shown in Figure 7-19. Chlorine is a gas at room temperature. Bromine is a liquid, but it evaporates easily. Iodine is a sofid that can change directly into a vapor. 

FIGURE 8.20 The Group 7A elements chlorine, bromine, ond iodine. Fluorine is o greenish-yellow gos thot ottocks ordinary glosswore. Astotine is radioactive. 

In the periodic table, the element hydrogen is sometimes grouped with the alkali metals (as in this book) and sometimes with the halogens. Explain why hydrogen can resemble the Group 1A and the Group 7A elements. 

The relatively large change in size in going from the first to the second member of a group also has important consequences for the Group 7A elements. 

What is the characteristic valence electron configuration of the group 7A elements, the halogens ... 

Some of the properties of the group 7A elements, the halogens, are given in TABLE 7.7. Astatine, vdiich is both extremely rare and radioactive, is omitted because many of its properties are not yet known. 

The hydrogen molecule, symbolized as H2, is called a diatomic molecule because it contains only two atoms. Other elements that normally exist as diatomic molecules are nitrogen (N2) and oxygen (O2), as well as the Group 7A elements—fluorine (F2), chlorine (CI2), bromine (Br2), and iodine (I2). Of course, a diatomic molecule can contain atoms of different elements. Examples are hydrogen chloride (HCl) and carbon monoxide (CO). 

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