Types of Decay for Various Isotopes

(Supplemental Notes for Chem 101, Haverford College, 1998, by Rob Scarrow).

The table designs below are based on Figure 22.5 of McMurray and Fay's General Chemistry (reference 1). The tables graphically show both stable isotopes (in green) and, for unstable isotopes, how the isotopes decay. The tables here were constructed based on information in reference 2. The tables are simplified because only the most probable decay pathways are shown (many isotopes decay by multiple pathways--for instance, ₇₀¹⁵⁶Yb decays by alpha emission 79% of the time, and by positron emission or electron capture [which are experimentally difficult to distinguish] 21% of the time).

legend: [blue] Beta Emission [green] Nonradioactive (stable) [yellow]Positron emission or electron capture [red] Alpha emission

From this graph it appears that

1. isotopes above and to the left of the stable isotopes decay with beta emission (this is generally true)
2. isotopes below and to the right of the stable isotopes decay with positron emission, electron capture, or alpha emission (this is generally true)
3. alpha emission is generally only found in isotopes far below and far to the right of the stable isotopes (this is not so always true, and in fact is only true for the elements with Z = about 70 to about 80, as shown).

Alpha emission in fact becomes quite rare for elements with less than 84 neutrons. This can be seen by extending the graph shown above to lower atomic numbers: legend: [blue] Beta Emission [green] Nonradioactive (stable) [yellow]Positron emission or electron capture [red] Alpha emission (Note: this table was constructed based on information in reference 2.).

Alpha emission becomes much more common for elements above Z = 84. legend: [blue] Beta Emission [green] Nonradioactive (stable) [yellow]Positron emission or electron capture [red] Alpha emission [purple] Spontaneous Fission (Note: this table was constructed based on information in reference 2.).

Decay series can be represented on the graph by lines. The graph here shows the decay series from 238U to 206Pb (contrast to Figure 22.6 in your textbook (ref. 1) legend: [blue] Beta Emission (N = -1, Z = +1) [green] Nonradioactive (stable) [yellow]Positron emission or electron capture (N = +1, Z = -1) [red] Alpha emission (N = -2, Z = +2) [purple] Spontaneous Fission (Note: this table was constructed based on information in reference 2.).

Another example (this one including positron emission or electron capture) is the decay series from 222U to 206Pb (note that the final product is the same as for the previous picture). legend: [blue] Beta Emission (N = -1, Z = +1) [green] Nonradioactive (stable) [yellow]Positron emission or electron capture (N = +1, Z = -1) [red] Alpha emission (N = -2, Z = +2) [purple] Spontaneous Fission (Note: this table was constructed based on information in reference 2.).

References

1. McMurray and Fay, "Chemistry", Prentice-Hall, 1995.

2. Lide, David R., ed. "Handbook of Chemistry and Physics", 71st edition, CRC Press, 1990.