Group 17 The Halogens

Compounds containing the halogens (Greek, halos + gen, salt former ) have been used since antiquity, with the first use probably that of rock or sea salt (primarily NaCl) as a food preservative. Isolation and characterization of the neutral elements, however, has occurred comparatively recently.  

All neutral halogens are diatomic and readily reduced to halide ions. All combine with hydrogen to form gases that, except for HR are strong acids in aqueous solution. Some physical properties of the halogens are summarized in Table 8.14. 

Element Ionization Energy (kJ mol Electron Affinity (kJ mol T Electro- negativity Melting Point (°C) Halogen Molecules, Xj X—X Boiling Distance Point ( O (pm) AH of Dissociation (kJ mol ) 

Source Data from Ionization energies cited in this chapter are from C. E. Moore, Ionization Potentials and Ionization Limits Derived from the Analyses of Optical Spectra, National Standard Reference Data Series, U.S. National Bureau of Standards, NSRDS-NBS 34, Washington, DC, 1970, unless noted otherwise. Electron affinity values listed in this chapter are from H. Hotop and W. C. Lineberger, J. Phys. Chem. Ref Data, 1985,14, 731. Standard electrode potentials listed in this chapter are from A. J. Bard, R. Parsons, and J. Jordan, Standard Potentials in A eous Solutions, Marcel Dekker (for lUPAC), New York, 1985. Electronegativities cited in this chapter are from J. B. Mann, T. L. Meek, and L. C. Allen, J. Am. Chem. Soc., 2000,122, 2780, Table 2. Other data are from N. N. Greenwood and A. Eamshaw, Chemistry of the Elements, Pergamon Press, Elmsford, NY, 1984, except where noted. 

Solutions of iodine in alcohol ( tincture of iodine ) are household antiseptics. Because of its radioactivity, astatine has not been studied extensively it would be interesting to be able to compare its properties and reactions with those of the other halogens. 

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Native copper has a distinctive reddish/brown color. Its first oxidation state (+1) forms compounds with copper ions named cuprous, also referred to as copper(I), and these ions are easily oxidized with elements in group 16 (e.g., oxygen and sulfur) and elements in group 17 (the halogens). 

Figure 8.1 Group 17 (the halogens) and Group 18 (the noble gases) of the periodic table are displayed here.

Periodic Trends in Bonding 22-2 Group 18 The Noble Gases 22-3 Group 17 The Halogens... 

The halogens, Group 17 (the column with F at the top), form ions with a -1 charge. 

In this section, the polyatomic cations of group 17 elements (halogen and interhalogen cations and polycations), cations and polycations of group 16 elements (O, S, Se, and... 

These reactions are typically limited to the halogens. The procedure for predicting the outcome of these reactions is the same as for the metals. The halogens also appear on the table of standard reduction potentials, but for reasons we will discuss in Chapter 17, the halogens get more reactive as you go up the table of reduction potentials. An easy way to remember the reactivities of the halogens is they are less reactive going down the group (as atomic number increases). 

Group lA (or 1) elements are also known as the alkali metals Group IIA (or 2), the alkaline earth metals Group VIIA (or 17), the halogens and Group VIIIA (or 18), the noble gases. 

Our last chance to view very active elements occurs in Group 7A(17). The halogens begin with fl uorine (F), the strongest electron grabber of all. Chlorine (Cl), bromine (Br), and iodine (I) also form compounds with most elements, and even rare astatine (At) is thought to be reactive [Group 7A(17) Family Portrait, p. 448]. 

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