Question
Explain how radioactive nuclei can emit $\beta-$particles even though atomic nuclei do not contain these particles? Hence explain why the mass number of radioactive nuclide does not change during $\beta-$decay?
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Answer
Radioactive nuclei do not contain electrons ($\beta-$particles), but $\beta-$particles are formed due to conversion of a neutron into a proton according to equation.
$^1_0\text{n}\ \rightarrow \ ^1_1\text{p}\ +\ _{-1} ^{0}\beta \ + \ \bar{ \upsilon}\\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ ^{\beta-\text{particle}}\ \ ^\text{antincutrino}$
The $\beta-$particle so formed is emitted at once. In this process one neutron is converted into one proton; so that the number of nucleons in the nucleus remains unchanged; hence mass number of the nucleus does not change during a $\beta-$decay.
$^1_0\text{n}\ \rightarrow \ ^1_1\text{p}\ +\ _{-1} ^{0}\beta \ + \ \bar{ \upsilon}\\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ ^{\beta-\text{particle}}\ \ ^\text{antincutrino}$
The $\beta-$particle so formed is emitted at once. In this process one neutron is converted into one proton; so that the number of nucleons in the nucleus remains unchanged; hence mass number of the nucleus does not change during a $\beta-$decay.
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