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General Chemistry Principles, Patterns, and Applications, 2011

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The halogens are so reactive that none is found in nature as the free element; instead, all but iodine are<br />

found as halide salts with the X − ion. Their chemistry is exclusively that of nonmetals. Consistent with<br />

periodic trends, ionization energies decrease down the group. Fluorine, the most reactive element in the<br />

periodic table, has a low F–F bond dissociation energy due to repulsions between lone pairs of electrons<br />

on adjacent atoms. Fluorine forms ionic compounds with electropositive elements <strong>and</strong> covalent<br />

compounds with less electropositive elements <strong>and</strong> metals in high oxidation states. All the halogens react<br />

with hydrogen to produce hydrogen halides. Except for F2, all react with water to form oxoacids, including<br />

the perhalic acids, which contain the halogens in their highest oxidation state. Halogens also<br />

form interhalogen compounds; the heavier halogen, with the lower electronegativity, is the central atom.<br />

K E Y T A K E A W A Y<br />

<br />

<br />

The halogens are highly reactive.<br />

All halogens have relatively high ionization energies, <strong>and</strong> the acid strength <strong>and</strong> oxidizing<br />

power of their oxoacids decreases down the group.<br />

C O N C E PTUAL P R OBLEMS<br />

1. The lightest elements of groups 15, 16, <strong>and</strong> 17 form unusually weak single bonds. Why are their bonds so<br />

weak?<br />

2. Fluorine has an anomalously low F–F bond energy. Why? Why does fluorine form compounds only in the −1<br />

oxidation state, whereas the other halogens exist in multiple oxidation states?<br />

3. Compare AlI 3 , InCl 3 , GaF 3 , <strong>and</strong> LaBr 3 with respect to the type of M–X bond formed, melting point, <strong>and</strong><br />

solubility in nonpolar solvents.<br />

4. What are the formulas of the interhalogen compounds that will most likely contain the following species in<br />

the indicated oxidation states: I (+3), Cl (+3), I (−1), Br (+5)?<br />

5. Consider this series of bromides: AlBr 3 , SiBr 4 , <strong>and</strong> PBr 5 . Does the ionic character of the bond between the Br<br />

atoms <strong>and</strong> the central atom decrease or increase in this series?<br />

6. Chromium forms compounds in the +6, +3, <strong>and</strong> +2 oxidation states. Which halogen would you use to produce<br />

each oxidation state? Justify your selections.<br />

7. Of ClF 7 , BrF 5 , IF 7 , BrF 3 , ICl 3 , IF 3 , <strong>and</strong> IF 5 , which one is least likely to exist? Justify your selection.<br />

A N S W E R S<br />

1. Electrostatic repulsions between lone pairs on adjacent atoms decrease bond strength.<br />

Saylor URL: http://www.saylor.org/books<br />

Saylor.org<br />

2059

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