mayurr12 said:
in nucleus
neutron--------------->proton & electron
electron is emited as B RAY WHILE proton is in nucleus then nucleus must carry +ve charge & it should b unstable but it is stable howwwww
The nucleus always carries a positive charge whether beta decay occurs or not. For instance, the nucleus of a nitrogen atom, contains 7 protons. That means it carries a charge of +7e and yet it is stable. I explain why in the next paragraph. On the other hand, the nucleus of a carbon atom contains 6 protons and carries a charge of +6e. It is stable as well. After the beta decay of a carbon nucleus, it becomes nitrogen with 7 protons in the nucleus. This nucleus is indistinguishable from any other nitrogen nucleus and is just as stable as they are.
The electromagnetic forces in the nucleus do tend to make it unstable. In that sense, you are right to expect that the nucleus might become more unstable after beta decay. Indeed, in some cases that is exactly what happens. However, there are other forces in the nucleus that are sufficient to overcome that tendency. These are called the weak force and the strong force.
Here are some examples of beta decay:
Radium 228 has a half life of close to 6 years. By beta decay it becomes actinium 228 with a half life of just over 6 hours. So as you expected, the nucleus become less stable.
Actinium 288 beta decays into Thorium 228 which has a half life of nearly two years. So in this case an unstable nucleus decays into one that is more stable.
I got these figures from this site:
http://en.wikipedia.org/wiki/Template:Thorium_series
Although beta decay does not necessarily render the nucleus unstable, it does turn an atom into a positive ion. The ion is unstable and will pick up an electron in what might be called a kind of decay (such language is very loose, and if the nucleus is in a molecule things get fuzzier). Perhaps this is what you meant.