Page 17 - Radiochemistry and nuclear chemistry

Radiochemistry and Nuclear Chemistry

Frequently, in such a chain, the half-life (tl/2) for the radioactive decay is shown either
above or below the arrow. A shorter notation is commonly used:

238 U ........ > 234Th ........ > 234pa ........ > 234 U ........ > 230 Yh, etc. (1.3)
4.5xlOgy 24 d 1.1 min 2.5xlOSy

where the half-lives are given in years (y) and days (d) 1. The three naturally occurring
radioactive decay series, which are known as the thorium series, the uranium series, and
the actinium series, are shown in Fig. 1.2. A fourth series, which originates in the synthetic
element neptunium, is also shown. This series is not found naturally on earth since all of
the radioactive species have decayed away long ago. Both the present symbolism of the
isotope as well as the historical (i.e. "radioelement') symbolism are given in Fig. 1.2. Note
that the rule of Fajans and Sod@ is followed in each series so that r causes a
decrease in atomic number by two units and mass number by four, whereas /3-decay
produces no change in mass number but an increase in atomic number by one unit.
Moreover, we see a pattern occurring frequently in these series where an a-decay step is
followed by two/3-decay steps. All known isotopes of elements 92 U to 81T1 are given in
Figure 5.1.

1.4. Atomic models

Neither radioactive decay nor the discovery of isotopes provided information on the
internal structure of atoms. Such information was obtained from scattering experiments in
which a substance, such as a thin metal foil, was irradiated with a beam of a-particles and
the intensity (measured by counting scintillations from the scattered particles hitting a
fluorescent screen) of the particles scattered at different angles measured (see Fig. 12.4).
It was assumed that the deflection of the particles was caused by collisions with the atoms
of the irradiated material. About one in 8000 of the a-particles was strongly deflected
through angles greater than 90 ~ Consideration of these rare events led Rutherford in 1911
to the conclusion that the entire positive charge of an atom must be concentrated in a very
small volume whose diameter is about 10-14 m. This small part of the atom he called the
nucleus. The atomic electrons have much smaller mass and were assumed to surround the
nucleus. The total atom with the external electrons had a radius of approximately 10-10 m
in contrast to the much smaller radius calculated for the nucleus.
It was soon shown that the positive charge of the atomic nucleus was identical to the
atomic number assigned to an element in the periodic system of Mendeleev. The
conclusion, then, is that in a neutral atom the small, positively charged nucleus was
surrounded by electrons whose number was equal to the total positive charge of the nucleus.
In 1913 N. Bohr, using quantum mechanical concepts, proposed such a model of the atom
which remains the basis of the modem atomic theory.

1 IUPAC recommends a for annum, instead of y, however y will be used throughout this text as it remains the
commonly used term.