Series
One Decay very useful way to display all the known
57
isotopes is to create a graph with the number of
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protons (Z) along the horizontal axis and the
number of neutrons, which we will give the
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symbol N, along the vertical axis. The stable
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elements form a thin, stair-stepping curve
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surrounded by a band of unstable elements. On this
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isotope graph, it can be seen that decay
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transmutes the isotope down and to the right ()
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while + decay moves it up and to the left ().
Z
40 41 42 43 44 45
The vast majority of the isotope graph thus looks
N
like the section shown at right. Notice that the
radioactive isotopes generally head along a diagonal line directly toward a stable isotope
(shown in black). The approximate half-life of each isotope is color-coded by the
background of each cell: red is less than one second; orange is less than one day; yellow
is less than one year (1a); green is less than 1000a; blue is less than 106a; and violet is
over a million years. Technetium (element 43) may not have any stable isotopes, but
some of them certainly last a long time.
We know that all of the isotopes with Z > 82 are unstable. Obviously, their radioactivity
must take them toward the stable elements, which are down and to the left. Our first guess
might suggest the emission of a particle that moves them one unit diagonally in that
direction. A bit of thought reveals that this would yield a deuterium nucleus (hydrogen-2)
as particle. the However, deuterium is very rare in nature. Clearly it does not like to form.
Instead, we find that alpha decay takes us down and left two units alone each axis. When
we examine the isotope graph for the heavy elements, we quickly discover that there are
four main paths by which most of the elements end up decaying down to a stable isotope.
The decays are always either or . Any heavy radioactive element will undergoes
certain decays that take it like a tributary toward these four large, interwoven rivers that
all head down the NZ graph toward stable isotopes of lead or, in the case of the last one,
thallium-205 (although this series passes through bismuth-209, which has a half-life so
incredibly long that we can say it ends there instead). These are called the radioactive
decay series, and are named after the highest common isotope that begins them.
The uranium-238 series (red squares on the NZ graph at right) goes ,
, , , , , , , , , , , , , Pb-206 (red circle). The uranium235 series (green) ends at Pb-207 (green circle). The thorium-232
series (blue) ends at Pb-208 (blue circle). The neptunium-237 series
(yellow) ends at Bi-209 or Tl-205 (yellow circle). Because some of
these decays are very slowU-238, for example, has a half-life of 4.5
billion yearsthe entire process can take a very long time. Since the
decays are random, a sample of U-238, for instance, will gradually
acquire a mixture of various isotopes along its decay series.