Untitled - Kelly Walsh High School

Nuclear Chemistry 295
Don’t Forget!
Electron capture is the only decay process we presented where you add a particle
on the left side of the reaction arrow.
20-2 Nuclear Stability
Sometimes it is difficult to predict if a particular isotope is stable and, if unstable,
what type of decay mode it might undergo. All isotopes that contain 84 or
more protons are unstable. These unstable isotopes will undergo nuclear decay.
For these large massive isotopes, we observe alpha decay most commonly.
Alpha decay gets rid of four units of mass and two units of charge, thus helping
to relieve the repulsive stress found in the nucleus of these isotopes. For other
isotopes of atomic number less than 83, we can best predict stability by the use
of the neutron to proton (n/p) ratio.
A plot of the # neutrons (n) versus the # protons (p) for the known stable isotopes
gives the nuclear belt of stability. (See your textbook for a figure of the
belt of stability.) At the low end of this belt of stability (Z < 20), the n/p ratio is 1.
At the high end (Z < 80), the n/p ratio is about 1.5. We can then use the n/p
ratio of the isotope to predict if it will be stable. If it is unstable, then the isotope
will utilize a decay mode that will bring it back onto the belt of stability.
Consider neon-18 or Ne-18. It has 10p and 8n, giving an n/p ratio of 0.8. For a
light isotope, like this one, this value is low. A low value indicates that this isotope
will probably be unstable. Neutron-poor isotopes, meaning that it has a
low n/p ratio do not have enough neutrons (or has too many protons) to be stable.
Decay modes that increase the number of neutrons and/or decrease the
number of protons are favorable. Both positron emission and electron capture
accomplish this by converting a proton into a neutron. As a rule, positron emission
occurs with lighter isotopes and electron capture with heavier ones.
Isotopes that are neutron-rich, that have too many neutrons or not enough protons,
lie above the belt of stability and tend to undergo beta emission because
that decay mode converts a neutron into a proton.
A particular isotope may undergo a series of nuclear decays until finally a stable
isotope forms. For example, radioactive U-238 decays to stable Pb-206 in
14 steps, half of these are alpha emissions and the other half are beta emissions.