rn

Explain how radioactive nuclei can emit beta particles even though atomic nuclei do not contain these particles. hence xplain why the mass no. of a radioactive nuclie does not change during beta decay?

Neutron in atomic nuclie consist of proton and electron.  It is assumed that beta-decay is conversion of neutron into proton and electron.  The beta decay occurs in case of nuclei which possess higher value of neutron to proton ratio than that for the stable nuclei. Beta particle is nothing but stream of electrons by beta decay. 

Atomic mass of electron is negligebily small so during the electron loss, the atomic mass of radioactive nuclei does not change whereas the atomic number increases by 1.