Question

how can radio acyive nucleii emit beta particle​

Answer 1

Hey mate....

Here is your answer....

Beta particles are nothing but electrons or positrons.

When in an unstable nucleus, the neutron gets converted to proton and electron along with liberation of antineutrino.

Or a proton might get converted to a neutron by liberating a positron along with neutrino.

You can ask you doubts in the comments.

-Mähëshwářàñ