DONNABOGNER Wichita State University Wichita, KS 67208
%eta Decay Diagram Roberl Suder The Science Academy of Austin LBJ High School 7309 Lazy Creek Lane Austin. TX 78724
The general chemistry curriculum usually contains a brief discussion of the nucleus of the atom. The topics covered include isotopes, nuclear stability, atomic structure, halflife, and nuclear decay, which is generally discussed in terms of nuclear equations. Alpha decay is usually easy for the students t o understand. The nucleus simply loses a particle consisting of two protons and two neutrohs, which causes the mass number to decrease by four units and the nuclear charge . t o decrease by two units. However, the understanding of beta decay can be more difficult because it is often hard for the student to comprehend how the loss of an electron from the nucleus of an atom can result in the gain of a proton and not affect the mass number. Students also question why the electron is in the nucleus in the first place, rather than orbiting a t some distance. Most textbooks explain beta decay in terms of balanced nuclear eauations, hut many students find this concept too abstract for understanding what is occurring in the nucleus. Also, they may erroneously conclude that orbiting electrons are somehowexpelled as a result of nuclear decay. Therefore, Idiagram the nucleus of the simplest atom that decays by beta emission, H-3 (Fig. 1).This isotope contains two neutrons and one proton, which accounts for a mass number of three. Also I show the neutron as consisting of both positive and negative areas, resulting in a neutral particle. When beta decay occurs, the neutron disintegrates permitting a series of unstable particles to escape. After the high-energy radiation has been given off, the two resulting stable particles are the proton and electron. The proton remains in the nucleus, resulting in the conversion of H-3 to He-3. The electron is ejected from the nucleus as the beta particle. In effect, the neutron has been changed to a proton by this process. Decay by electron capture can be shown in a similar manner (Fig. 2). In this case, an orbitingelectron is drawn into the nucleus, converting a proton into a neutron. This results in a decrease of the nuclear charge, but leaves the mass number unchanged. The simplest atom that decays by elec-
tron capture is Be-7, which contains four protons and three neutrons. When an inner orbiting electron is drawn into the nucleus, a proton is changed to a neutron, converting the Be7 into Li-7. Gamma rays are also produced during electron capture as a means of releasing excess energy. Too often instructors believe that students can intuitively understand nuclear decay from balanced equations, but i t has been my experience that a diagram greatly enhances their knowledge of this process.
H-3 nucleus mass number = 3 nuclear cnarqe = I
He-3 nucleus lass number = 3 nuclear charge = 2 8 particle
:H
--+ 32 H e
8
+-,e
Figure 1. Diagram showing &cay of t t 3 by beta emission.
EC DECRY
0RP-7 nucleus nass number = 7 neclear cbarge = 4 :6e
+.!e
-
5
LI-7nucleus mass number = 7 nuclear charge = j 11
Figure 2. Diagram showing decay of 88-7 by electron capture.
Volume 66
Number 3
March 1989
231
