Explain $\beta -$ decay and how does a radioactive nucleus emit $\beta -$ particles, even if there are no $\beta -$ particles in the nucleus ? Here why does radioactive nuclide not change during beta emission ? 

Process in which a nucleus spontaneously emits an electron ( $\beta^{-}$decay) or a positron( $\beta^{+}$decay) is called $\beta$-decay.

$\alpha$-decay and $\beta$-decay are spontaneously processes with certain disintegration energy and half life

and are a statistical processes obeyed the radioactive law.

Such a decay is characterize by half life $\mathrm{T}_{1 / 2} .$ The emission of electron in $\beta^{-}$decay is accompanied by the emission of an antineutrino $(\bar{v})$ in $\beta^{+}$decay, while neutrino (v) is also emitted along with positron in $\beta^{+}$decay.

Neutrinos have very small (even zero) mass compared to electrons. They have only weak interaction with other particles. They are therefore very difficult to detect.

Neutrinos and antineutrons can penetrate large quantity of matter (even earth) without any interaction.

$\beta^{-}$decay : In $\beta^{-}$decay, atomic mass number of daughter nucleus remain same that of parent nucleus but the atomic number of the nucleus goes up by 1 . Its general equation,

${ }_{\mathrm{Z}}^{\mathrm{A}} \mathrm{X} \rightarrow{ }_{\mathrm{Z}+1}^{\mathrm{A}} \mathrm{Y}+{ }_{-1} e^{0}\left(\beta^{-}\right)+\overline{\mathrm{v}}$

$\text { where } \quad \mathrm{X}=\text { parent nucleus }$

$\qquad$

$\mathrm{Y}=\text { daughter nucleus }$

${ }_{-1} e^{0}=\beta^{-} \text {particle }$

$v=$ anti particle of positron neutrino $\rightarrow$ it has no mass and charge but has spin $\pm \frac{1}{2}$

In $\beta$-decay a neutron is converted into a proton or a proton is converted into a neutron and the masses of proton and neutron is almost equal so the mass number of nuclide $A=Z+N$ does not change.