why lithium reacts less vigarously than sodium ?

Dohhh !! check the reactivity series

http://en.wikipedia.org/wiki/Reactivity_series

Elements at the top are hyper reactive and this decreases downwards..

sodium comes above lithium so likewise it reacts more vigorously

Lithium is a small molecule compared to sodium.... the attraction between the outermost electron and the nucleus in Li is more than the attraction between outermost electron and nuclei of Na... when a small amount of energy is supplied to Sodium, the outermost electron jumps out and enables Na to take part in Chemical reactions... while in the case of Li more amount of energy is to be supplied to overcome the attraction between the outermost electron and the nucleus... Hence Na reacts vigorously when compared to Li..... PLZ approve this answer!!!!!

but energy will be released on rection with water rt

U will learn in higher classes that energy is inversely propotional to stability.....

consider the reaction:

REACTANTS ----> PRODUCTS

where enormous amount of energy is lost in the form of heat... the energy of the products is less than the energy of the reactants (heat is lost)... hence the products is stable when compared to the reactants... Na has higher energy than Li... it is less stable compare to Li... it reacts with water vigorously and enormous amount of heat is lost during the process and forms NaOH which is stable... Li also reacts with water vigorously but since its already stable (has less energy) it reacts less vigorusly than water....