The first reaction was a β+ decay (positron emission) that must have looked like:
Again, balance the atomic numbers:
Z = 28 + 1 = 29
You carry out a balancing of mass numbers by taking into account that a proton has disappeared on
the left and reappeared as a neutron on the right, leaving mass number unchanged:
A = 64 + 0 = 64
By looking at the periodic table you find that Z = 29 means that X is Cu. Since A = 64, that means that
the solution is:
While the problem did not ask for it, it is possible again to look at the periodic table to find that Z’ =
Z” = 28 means Y * = Y = Ni.
Electron Capture. Certain unstable radionuclides are capable of capturing a core electron that
combines with a proton to form a neutron. The atomic number is now one less than the original, but
the mass number remains the same. Electron capture is a rare process that is perhaps best thought
of as an inverse β– decay.
Radioactive Decay Half-Life (τ1/2)
In the chapter on kinetics, we discussed radioactive decay as an example of first-order kinetics, and
introduced the concept of the half-life (τ1/2): In a collection of a great many identical radioactive
isotopes, the half-life of the sample is the time it takes for half of the sample to decay.
Example: If the half-life of a certain isotope is 4 years, what fraction of a sample of that isotope
will remain after 12 years?