Page 295 - Theory and Problems of BEGINNING CHEMISTRY

284 NUCLEAR REACTIONS [CHAP. 19

19.5. NUCLEAR FISSION AND FUSION
Nuclear fission refers to splitting a (large) nucleus into two smaller ones, plus one or more tiny particles
listed in Table 19-3. Nuclear fusion refers to the combination of small nuclei to make a larger one. Both types of
processes are included in the term artificial transmutation.

Table 19-3 Nuclear Projectiles and Products ∗

Nuclear Rest Mass
Name Symbol Identity (amu)

Proton 1 Horp Hydrogen nucleus 1.00728
Deuteron 2 Hord Heavy hydrogen nucleus 2.0135
Tritium 3 H Tritium nucleus 3.01550
Helium-3 3 He Light helium nucleus 3.01493
Neutron 1 norn Free neutron 1.008665
Alpha α Helium nucleus 4.001503
Beta β High-energy electron 0.00054858
Gamma γ High-energy light particle 0.0
Positron + β Positive electron 0.00054858

∗ 12
Larger projectiles are identified by their regular isotopic symbols, such as C.
6
Transmutation means converting one element to another (by changing the nucleus). The first artificial
transmutation was the bombardment of 14 N by alpha particles in 1919 by Lord Rutherford.
7
14 4 17 1
1
2
7 N + He −→ 8 O + H
The alpha particles could be obtained from a natural decay process. At present, a variety of particles can be used
to bombard nuclei (Table 19-3), some of which are raised to high energies in “atom smashing” machines. Again,
nuclear equations are written in which the net charge and the total of the mass numbers on one side must be the
same as their counterparts on the other side.

EXAMPLE 19.8. What small particle(s) must be produced with the other products of the reaction of a neutron with a 235 U
92
nucleus by the following reaction?
235 1 90 143 Xe + ?
92 U + n −→ 38 Sr + 54
0
Ans. In order to get the subscripts and the superscripts in the equation to balance, the reaction must produce three neutrons:
235 1 90 143 Xe + 3 n
1
0
38
92 U + n −→ Sr + 54 0
This reaction is an example of a nuclear chain reaction, in which the products of the reaction cause more
of the same reaction to proceed. The three neutrons can, if they do not escape from the sample first, cause three
more such reactions. The nine neutrons produced from these reactions can cause nine more such reactions, and
so forth. Soon, a huge number of nuclei are converted, and simultaneously a small amount of matter is converted
to a great deal of energy. Atomic bombs and nuclear energy plants both run on this principle.

EXAMPLE 19.9. If each neutron in a certain nuclear reaction can produce three new neutrons, and each reaction takes
1 s, how many neutrons can be produced theoretically in the 15th second?
Ans. Assuming that no neutrons escaped, the number of neutrons produced during the 15th second is
15
3 = 14 348 907
28
The number produced in the 60th second is 4.24 × 10 .