Page 190 - Essentials of physical chemistry

152 Essentials of Physical Chemistry

model of the nucleus, was partially rectified when the United States awarded her the U.S. Fermi
Prize in 1966 jointly with Hahn and his assistant Fritz Strassmann. Meitner and her nephew coined
the term ‘‘fission’’ to describe the splitting of the uranium nucleus and today she is known as the
‘‘Mother of Fission.’’ Meitner refused to participate in the Manhattan Project because she did not
want to work on a bomb. Since that time the kinetic scheme we have described as A ! B ! C has
been extended to many cases of successive first-order decay by radioactive isotopes. Many of the
heavier elements are in fact radioactive but with long half-lives and they eventually decay to some
stable isotope of Pb in many cases. In retrospect, what Hahn was observing was probably a small
amount of a nuclear reaction:

235 1 141 92 Kr þ 3 n:
1
92 U þ n ! 56 Ba þ 36 0
0
Since Kr is a gas, only Ba would show up in the analysis of the solids in the U target. It is important
to note that this reaction produces more neutrons than are needed to start the reaction so a chain
reaction is definitely possible in the presence of more 235 U.
92
In the late 1950s and early 1960s, undergraduate science education in the United States often
included a course in ‘‘radiochemistry.’’ This author took such a course which included a set of
laboratory experiments using radioactivity counters and careful chemistry experiments with radio-
active isotopes. Today such courses seldom occur in chemistry curricula due to the dangers of
radioactivity. However, this author believes physical chemistry is possibly the last opportunity for
some undergraduate science majors to gain a minimal understanding of nuclear chemistry which is
still important in the present age. In the absence of a potentially dangerous laboratory course, we can
at least include some paper-and-pencil examples here. In a wonderful general chemistry text by
G. Sasin and R. Sasin [10] (reprinted as a soft cover course-pack later by this author [11]), there is a
very short chapter on ‘‘nuclear chemistry.’’ This material was of great interest in the 1950s.
Although Irene Joliot-Curie (daughter of Marie Curie) had won a Nobel Prize in 1935 (in chemistry)
for making aluminum radioactive due to bombardment with alpha particles, public interest peaked
following WWII. The text by Sasin and Sasin is usefully succinct, so we include some of their
material to give a sense of the way in which the alchemist dream of changing one element to another
can occur with modern facilities in nuclear reactors and particle accelerators.
In the examples of nuclear reactions which follow we will focus on conservation of particles
2
even though we know from Einstein’s formula E ¼ mc that small amounts of mass can be
converted into enormous amounts of energy and we will use integers for particle numbers. In this
way, we have to balance the isotope superscripts as well as the subscripts. Note in the fission of
235
92 U, the number of (protons þ neutrons) in the superscript adds up to 236 on both sides of the
reaction and the number of protons in the subscripts adds up to 92 on both sides of the reaction. Here
are a few nuclear reactions given in the Sasin and Sasin text [10,11] (by permission):

A. a-Particle bombardment
1. Emission of a proton, a process common to many elements from C to K

1
27 4 30 Si þ H:
13 Al þ He ! 14 1
2
2. Emission of a neutron
11 4 14 1
0
5 B þ He ! 7 N þ n:
2
B. Proton bombardment
7 Li þ H ! 2 He:
1
4
3 1 2