18.4 Structure and general properties of the nonmetals  (Page 2/8)

The variety of oxidation states displayed by most of the nonmetals means that many of their chemical reactions involve changes in oxidation state through oxidation-reduction reactions. There are five general aspects of the oxidation-reduction chemistry:

1. Nonmetals oxidize most metals. The oxidation state of the metal becomes positive as it undergoes oxidation and that of the nonmetal becomes negative as it undergoes reduction. For example:
   
   $\begin{array}{cccc}\text{4Fe}(s)+\hfill & {\text{3O}}_2(g)\hfill & ⟶\hfill & {\text{2Fe}}_2{\text{O}}_3(s)\hfill \\ 0\hfill & 0\hfill & & \mathrm{+3}\mathrm{-2}\hfill \end{array}$
2. With the exception of nitrogen and carbon, which are poor oxidizing agents, a more electronegative nonmetal oxidizes a less electronegative nonmetal or the anion of the nonmetal:
   
   $\begin{array}{cccc}\text{S}(s)+\hfill & {\text{O}}_2(g)\hfill & ⟶\hfill & {\text{2SO}}_2(s)\hfill \\ 0\hfill & 0\hfill & & \mathrm{+4}\mathrm{-2}\hfill \end{array}$
   $\begin{array}{cccc}{\text{Cl}}_2(g)+\hfill & {\text{2I}}^{\mathrm{-}}(aq)\hfill & ⟶\hfill & {\text{I}}_2(s)+{\text{2Cl}}^{\mathrm{-}}(aq)\hfill \\ 0\hfill & & & 0\hfill \end{array}$
3. Fluorine and oxygen are the strongest oxidizing agents within their respective groups; each oxidizes all the elements that lie below it in the group. Within any period, the strongest oxidizing agent is in group 17. A nonmetal often oxidizes an element that lies to its left in the same period. For example:
   
   $\begin{array}{cccc}\text{2As}(s)+\hfill & {\text{3Br}}_2(l)\hfill & ⟶\hfill & {\text{2AsBr}}_3(s)\hfill \\ 0\hfill & 0\hfill & & \mathrm{+3}\mathrm{-1}\hfill \end{array}$
4. The stronger a nonmetal is as an oxidizing agent, the more difficult it is to oxidize the anion formed by the nonmetal. This means that the most stable negative ions are formed by elements at the top of the group or in group 17 of the period.
5. Fluorine and oxygen are the strongest oxidizing elements known. Fluorine does not form compounds in which it exhibits positive oxidation states; oxygen exhibits a positive oxidation state only when combined with fluorine. For example:
   
   $\begin{array}{cccc}{\text{2F}}_2(g)+\hfill & {\text{2OH}}^{\mathrm{-}}(aq)\hfill & ⟶\hfill & {\text{OF}}_2(g)+{\text{2F}}^{\mathrm{-}}(aq)+{\text{H}}_2\text{O}(l)\hfill \\ 0\hfill & & & \mathrm{+2}\mathrm{-1}\hfill \end{array}$

With the exception of most of the noble gases, all nonmetals form compounds with oxygen, yielding covalent oxides. Most of these oxides are acidic, that is, they react with water to form oxyacids. Recall from the acid-base chapter that an oxyacid is an acid consisting of hydrogen, oxygen, and some other element. Notable exceptions are carbon monoxide, CO, nitrous oxide, N ₂ O, and nitric oxide, NO. There are three characteristics of these acidic oxides:

1. Oxides such as SO ₂ and N ₂ O ₅ , in which the nonmetal exhibits one of its common oxidation states, are acid anhydrides and react with water to form acids with no change in oxidation state. The product is an oxyacid. For example:
   
   ${\text{SO}}_2(g)+{\text{H}}_2\text{O}(l)⟶{\text{H}}_2{\text{SO}}_3(aq)$
   ${\text{N}}_2{\text{O}}_5(s)+{\text{H}}_2\text{O}(l)⟶{\text{2HNO}}_3(aq)$
2. Those oxides such as NO ₂ and ClO ₂ , in which the nonmetal does not exhibit one of its common oxidation states, also react with water. In these reactions, the nonmetal is both oxidized and reduced. For example:
   
   $\begin{array}{ccccc}{\text{3NO}}_2(g)+\hfill & {\text{H}}_2\text{O}(l)\hfill & ⟶\hfill & {\text{2HNO}}_3(aq)+\hfill & \text{NO}(g)\hfill \\ \mathrm{+4}\hfill & & & \mathrm{+5}\hfill & \mathrm{+2}\hfill \end{array}$
   
   Reactions in which the same element is both oxidized and reduced are called disproportionation reactions .
3. The acid strength increases as the electronegativity of the central atom increases. To learn more, see the discussion in the chapter on acid-base chemistry.

The binary hydrogen compounds of the nonmetals also exhibit an acidic behavior in water, although only HCl, HBr, and HI are strong acids. The acid strength of the nonmetal hydrogen compounds increases from left to right across a period and down a group. For example, ammonia, NH ₃ , is a weaker acid than is water, H ₂ O, which is weaker than is hydrogen fluoride, HF. Water, H ₂ O, is also a weaker acid than is hydrogen sulfide, H ₂ S _, which is weaker than is hydrogen selenide, H ₂ Se. Weaker acidic character implies greater basic character.