21.3 Radioactive decay  (Page 8/21)

α (helium nuclei), β (electrons), β ⁺ (positrons), and η (neutrons) may be emitted from a radioactive element, all of which are particles; γ rays also may be emitted.

(a) conversion of a neutron to a proton: ${}_0^1\text{n}⟶{}_1^1\text{p}+{}_{\mathrm{+1}}^0\text{e};$ (b) conversion of a proton to a neutron; the positron has the same mass as an electron and the same magnitude of positive charge as the electron has negative charge; when the n:p ratio of a nucleus is too low, a proton is converted into a neutron with the emission of a positron: ${}_1^1\text{p}⟶{}_0^1\text{n}+{}_{\mathrm{+1}}^0\text{e};$ (c) In a proton-rich nucleus, an inner atomic electron can be absorbed. In simplest form, this changes a proton into a neutron: ${}_1^1\text{p}+{}_{-1}^0\text{e}⟶{}_0^1\text{p}$

The electron pulled into the nucleus was most likely found in the 1 s orbital. As an electron falls from a higher energy level to replace it, the difference in the energy of the replacement electron in its two energy levels is given off as an X-ray.

Manganese-51 is most likely to decay by positron emission. The n:p ratio for Cr-53 is $\frac{29}{24}$ = 1.21; for Mn-51, it is $\frac{26}{25}$ = 1.04; for Fe-59, it is $\frac{33}{26}$ = 1.27. Positron decay occurs when the n:p ratio is low. Mn-51 has the lowest n:p ratio and therefore is most likely to decay by positron emission. Besides, ${}_{24}^{53}\text{Cr}$ is a stable isotope, and ${}_{26}^{59}\text{Fe}$ decays by beta emission.