The emission of an α-particle from the nucleus of a radioactive substance is known as α-particle change. Since an α-particle carries two units of positive charge and four units of mass; the new element formed on the emission of α-particle (daughter element) has an atomic number less by two units and atomic mass less by four units than the parent element. Thus, the daughter element is displaced two groups to the left of its parent element in the Periodic table. Example- Ra (atomic number = 88, atomic mass = 226) is present in group 2 of the periodic table and when it loses an α-particle it changes into Radon (atomic number = 86, atomic mass = 222) which is in the 18th group.
The ejection of an electron in the form of a β-particle from the nucleus of a radioactive element is known as β-particle change. Since there are no electrons (β-particle) in the nucleus, it is believed that at first, a neutron breaks down into a proton and an electron and then the Proton is retained by the nucleus while the electron leaves the nucleus in the form of a β-particle i.e.
0n1 (neutron) ———> 1H1 (proton) + -1e0 (electron)
Since the mass of the β-particle is negligible and so the new element formed on the expulsion of a β-particle (daughter element) will have the same atomic mass but its atomic number increases by one unit (due to the addition of one more proton in the nucleus) than the parent element. Thus, the daughter element is displaced one group to the right of its parent element in the periodic table. Example- Radioactive Bismuth (atomic number = 83, atomic mass = 211) present in group 15 of the periodic table loses β-particle to form Polonium (atomic number = 84, atomic mass = 211) which is present in 16th group i.e.
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