Explain the formation of metal hydrides from hydrogen.

• Metals do not generally react with hydrogen under normal conditions.
• At high temperatures, highly reactive metals (sodium, potassium, calcium, etc) react with hydrogen to form corresponding metal hydride.
• The reaction involved is:

$$\begin{gathered} \underset{\mathrm{Hydrogen}}{{\mathrm{H}}_2}+\underset{\mathrm{Potassium}}{2\mathrm{K}}\stackrel{∆}{\to }\underset{\mathrm{Potassium}\mathrm{hydride}}{2\mathrm{KH}} \\ \underset{\mathrm{Hydrogen}}{{\mathrm{H}}_2}+\underset{\mathrm{Calcium}}{\mathrm{Ca}}\stackrel{∆}{\to }\underset{\mathrm{Calcium}\mathrm{hydride}}{{\mathrm{CaH}}_2} \end{gathered}$$