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Compounds that contain hydrogen bound to a nonmetal are called nonmetal hydrides. Because they contain hydrogen in the +1 oxidation state, these compounds can act as a source of the H+ ion in water.
Metal hydrides, on the other hand, contain hydrogen bound to a metal. Because these compounds contain hydrogen in a -1 oxidation state, they dissociate in water to give the H- (or hydride) ion.
The H- ion, with its pair of valence electrons, can abstract an H+ ion from a water molecule.
Since removing H+ ions from water molecules is one way to increase the OH- ion concentration in a solution, metal hydrides are bases.
A similar pattern can be found in the chemistry of the oxides formed by metals and nonmetals. Nonmetal oxides dissolve in water to form acids. CO2 dissolves in water to give carbonic acid, SO3 gives sulfuric acid, and P4O10 reacts with water to give phosphoric acid.
Metal oxides, on the other hand, are bases. Metal oxides formally contain the O2- ion, which reacts with water to give a pair of OH- ions.
Metal oxides therefore fit the operational definition of a base.
We see the same pattern in the chemistry of compounds that contain the OH, or hydroxide, group. Metal hydroxides, such as LiOH, NaOH, KOH, and Ca(OH)2, are bases.
Nonmetal hydroxides, such as hypochlorous acid (HOCl), are acids.
The table below summarizes the trends observed in these three categories of compounds. Metal hydrides, metal oxides, and metal hydroxides are bases. Nonmetal hydrides, nonmetal oxides, and nonmetal hydroxides are acids.
Typical Acids and Bases
The acidic hydrogen atoms in the non-metal hydroxides in the table above aren't bound to the nitrogen, sulfur, or phosphorus atoms. In each of these compounds, the acidic hydrogen is attached to an oxygen atom. These compounds are therefore all examples of oxyacids.
Skeleton structures for eight oxyacids are given in the figure below. As a general rule, acids that contain oxygen have skeleton structures in which the acidic hydrogens are attached to oxygen atoms.