Group I of the Periodic Table is composed of highly reactive metals. They react vigorously with water to produce hydroxides and release hydrogen. Even in air they react quickly to cover their surfaces with oxides. In nature they are found in ionic compounds, but never in the pure state. Their chemical activity increases as you move downward through the periods from Li to Na to K, etc.
Alkali metals tend to lose one electron and form ions with a single positive charge. They form ionic compounds (salts) in reaction with the halogens (alkali halides). Sodium and potassium ions form important constituents of body fluids (electrolytes).