Why alkali metals are so vigorous and exothermic that  the hydrogen evolved catches fire.??

Dear Student,
Alkali are highly reactive metals .The reactivity of alkali metals increases as we go down the group. This high reactivity is attributed to the electropositive nature of the metals . Alkali metals have a tendency to lose the electron in valence shell to achieve a stable octet. Alkali metals react explosively with water forming ionized metal , with evolution of H₂gas. The reaction is accompanied by release of large amount of heat, Steam formation and ignition of hydrogen gas formed , thts why reaction is vigorous,
At the bottom of the group are the most reactive  metals ,
Sodium on top portion of the group  reacts with water melts and forms a ball that floats on the surface. The metal darts on the surface because of the heat released. Hydrogen gas evolved ignites when Na metal is trapped on the walls of the container. 
Caesium reacts violently and causes the bursting of the container .
There are three types of enthlpy changes during reaction of alkali metal and water. $$\begin{gathered} X\left(metal\right)\to X\left(g\right) ∆H=+ve[atomozation energy] \\ x\left(g\right)+e^{-}\to X^{+}\left(g\right)∆H=+ve[ionization energy] \\ {X}^{+}\left(g\right)\to X^{+}\left(aq\right)∆H= -ve \left(large\right)[hydration energy] \\ Overall ∆H=-ve \end{gathered}$$
Due to large amount of heat released reaction is vigorous
Regards!