@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