Why does sodium explode in water?

Sodium is a highly reactive alkali metal because it has just one electron in its outer shell, which it gives up readily. When sodium touches water, it forms sodium hydroxide and hydrogen gas, releasing so much heat that the hydrogen may catch fire or explode. This is a classic demonstration in chemistry laboratories, though it is dangerous and must be done only under controlled conditions. Sodium’s reactivity is one reason it is not found free in nature and instead occurs in compounds such as sodium chloride, common table salt.