Page 292 - Theory and Problems of BEGINNING CHEMISTRY

CHAP. 19] NUCLEAR REACTIONS 281

Table 19-1 Products of Natural Radioactivity

Symbol Name Mass Number Charge Identity

α Alpha particle 4 2+ Helium nucleus
β Beta particle 0 1− High-energy electron
γ Gamma ray 0 0 High-energy particle of light

Mass number

4 a 0 b 0 g
2 −1 0

Charge
The superscripts refer to the mass numbers of the particles; the subscripts refer to their charges.
Nuclear equations are written with both the total charge and the total of the mass numbers unchanged from
reactants to products. That is, the total of the subscripts of the reactants equals the total of the subscripts of the
products, and the total of the superscripts of the reactants equals the total of the superscripts of the products. The
subscripts of isotopes may be omitted because the symbol of the element gives the atomic number.

EXAMPLE 19.1. Show that the mass number and the total charge are both conserved in the natural disintegration of 238 U:
92
238 234
U −→ Th + α
Ans. The equation may be rewritten including all atomic numbers and mass numbers:
4
238 U −→ 234 Th + He or 238 U −→ 234 Th + α
4
92 90 2 92 90 2
Adding the 234 + 4 superscripts of the products gives the superscript of the reactant. Adding the 90 + 2 subscripts
of the products gives the subscript of the reactant. The nuclear equation is balanced.

EXAMPLE 19.2. Complete the following nuclear equation:
233 233
Pa −→ U + ?
Ans. Inserting the proper subscript and superscript indicates that the product is a particle with 1− charge and 0 mass
number:
233 233 0 ?
91 Pa −→ 92 U + −1
The missing particle is a beta particle (Table 19-1).
233 233 0 β
91 Pa −→ 92 U + −1
Note that a beta particle has been emitted from the nucleus. This change has been accompanied by the increase in
the number of protons by 1 and a decrease in the number of neutrons by 1. In effect, a neutron has been converted
to a proton and an electron, and the electron has been ejected from the nucleus.

The emission of a gamma particle causes no change in the charge or mass number of the original particle.
(It does cause a change in the internal energy of the nucleus, however.) For example,

119 119 0
0
50 Sn −→ 50 Sn + γ
The same 119 Sn isotope is produced, but it has a lower energy after the emission of the gamma particle.
50