Originally Posted by Nannan
I have an doubt
How can an electron keep revolving without falling into nucleus by losing energy?
Some one please explain
Sounds like you are discussing the Bohr model of the atom?
In the early 1900s, when Bohr first worked out the general construction of the atom there was no reason that the electron shouldn't fall into the nucleus. It was an assumption that had to be put into the theory. The rule is kind of ad-hoc and has no other purpose than to try to sweep the issue under the proverbial carpet.
Basically the idea is that, just as happens with the planets, we can have the electrons orbiting the nucleus. This is formalized by the de Broglie wavelength (another major accomplishment). It says that the electron is a wave and in order to make any sense has to orbit it has to go around in a coherent fashion: it has to end up where it began or the wave would go away via destructive interference.
So if we have an electron with a wavelength (lambda) then the radius of the electron's orbit would have to be n(lambda) = 2 (pi) r, where n is a positive integer.