@ananth...he is actually asking how u got this... :)

just apply electrostatics yaar..

dekho...
electron revolves..there must be some force..thte only force tht is responsible is electrostatic..
thus centripetal force = electrostatic force...

now ..by Bohr's model u know tht angular momentum is nh/2pi...

use these informations to get ur velocity..

 

KZe² /  r²   =  mv² / r ...  

 

form here u get

 

KZe²  =  mv²r...

 

put mvr = nh / 2pi..

 

u get v =  [2pi K e² / h ] . Z/n ..

 

thing closed by brackets turns out to be ..2.18 x 10⁶ m/s..

 

     

 

u get ..KZe²  =  mv²r...

 

now put  mvr = nh / 2