Why are alkali metals so reactive?

Alkali metals are reactive because they are highly electropositive. Electropositivity is the property of an atom to lose its electrons. Metals (especially alkali and alkaline earth metals) tend to lose electrons easily i.e. they have very low electronegativities.

Alkali metals when come in contact with water, react vigorously. Some alkali metals may cause an explosion even at room temperatures. This chemical reactivity is due to;

  1. The unique (ns1) outermost shell electronic configuration.
  2. Very low first ionization energies.

They can easily lose valence electrons to attain a neighboring noble gas stable configuration. They are called alkali metals because they form water-soluble bases upon reaction with water.

The alkali metals are so reactive that they are usually found in combined states in nature i.e, ores of salts. For example, Halite (NaCl), Sylvite (KCl), and Carnallite (KCl·MgCl2), etc.

Reactions of Alkali metals with water

The reactivity of alkali metals increases as we go down the group. Same is the case while reacting with water. Almost all alkalis react with water.

Lithium in water

2Li(s) + 2H2O(aq)  →  2LiOH(aq) + H2(g)

Lithium fizzes when added to water and eventually vanishes.

Sodium in water

2Na(s) + 2H2O(aq)  →  2NaOH(aq) + H2(g)

Sodium when added to water melts and floats around the surface. It fizzes vigorously i.e. it is a highly exothermic reaction such that the H2 gas burns due to excess heat energy to form H2O as soon as it gets formed. 

Potassium in water

2K(s) + 2H2O(aq)  →  2KOH(aq) + H2(g)

Potassium when added to water melts and floats too. The hydrogen burns simultaneously. There is even an explosion at the end.

Similarly, other higher alkalis form an alkaline solution along with the production of heat energy and hydrogen gas. These solutions are good chemical indicators and are strongly basic.

Although alkali metals are corrosive too. This must be kept in mind while doing the above reactions.