In its pure form, sodium is a soft, silvery-white metal that is highly reactive. It is a member of the alkali metals group, which means that it has one valence electron and is very easy to oxidize. Sodium is so reactive that it must be stored in a dry, airtight environment to prevent it from reacting with oxygen and moisture in the air. When exposed to air, sodium quickly forms a dull oxide layer on its surface, which protects it from further oxidation.