[karanmaster0007]

The orbitals in an atom must occur at certain distances from the nucleus to create a stable atom. At these distances, the orbitals allow the electron wave to complete one or more half-wavelengths (y, 1, 1y, 2, 2y, and so on) as it travels around the nucleus. The electron wave can then double back on itself and constructively interfere with itself in a way that reinforces the wave. Any other distance would cause the electron to interfere with its own wave in an unpredictable and unstable way, creating an unstable atom.
Microsoft ® Encarta ® 2008. © 1993-2007 Microsoft Corporation. All rights reserved.
 

if u help me god will you and i will also, surely 

[renge ishyo]

The easiest way to understand this is to consider the de broglie wave picture of the atom. What they are saying is that the electron wave must "connect" with itself on its return trip around the atom. Consider the following picture taken from http://www.kutl.kyushu-u.ac.jp/seminar/MicroWorld2_E/2Part1_E/2P11_E/deBroglie_wave_E.htm:



It shows that this is not an allowed orbit for the electron because the wave does not connect with itself at the top of the picture. Hence, you have to increase or decrease the size of the orbit so that the wave can connect with itself all the way around as in the picture from http://www.clickandlearn.org/chemistry/DeBroglie.htm:



The descriptions on either site will provide more information for you on these matters.

[karanmaster0007]

thnx for explanation. i got it.