5. Gain and Loss of Electrons   Previous PageNext Page
       Oxygen Compounds: Acids or Bases?

About 60% of the Earth's crust, 20% of the air we breathe, and 26% of all living matter are made up of oxygen atoms. How the different elements react when brought close to oxygen is an important part of their chemical behavior. Water, the medium in which life evolved, can be thought of as a medium for bringing oxygen atoms close to other substances.

All of the elements that we have discussed so far, with the exception of helium and neon, form compounds with oxygen, called oxides. Oxides of metals are bases, and oxides of nonmetals are acids. The reason for this difference in behavior lies in the electronegativities of the atoms bound to oxygen.

Oxides of metals are bases

Oxides of nonmetals are acids


The second-shell elements run the gamut of electronegativity from 1.0 (Li) to 4.0 (F). When oxides of these elements are dissolved in water, bonds are formed of the general type

in which X is a second-shell element. (Outer-shell electron pairs on oxygen atoms will be shown explicitly in the following discussion. Remember that a chemical bond line as in X-O also represents an electron pair.)

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