Doubt in Radioactivity

The anti-neutrino is the anti-particle brother of the neutrino. It is released in B^- decay. It accompanies the electron as a chargeless, massless particle. (a neutrino is released in corresponding B⁺ decay)
It was found that somehow the phenomenon of beta decay was violating the energy conservation principle, as the end point(after decay) energy was far less than it was supposed to be. Thus the neutrino/anti-neutrino hypothesis was put forward.
The sum of energies of particles released thus is the same as that of the energy stored by the parent nucleus.

To read about neutrinos, you can check out Wikipedia. It has lots of stuff on it. Search for "Electron Anti-neutrino" to specifically check out beta decay.

naturally it isnt possible , but it can be done artificially

Soham, All these decays take place in nucleus and not in molecules as you have mentioned