A simple model for establishing the Bohr and Wheeler limit for Z2/A against spontaneous fission is developed. The physics involved is sufficiently simple to make the model suitable for sophomore students. Beyond showing how fundamental energy considerations lead to a limit for Z2/A, the model helps to make clear the physical origin of the numerical values of the surface and Coulomb terms in the semiempirical mass formula. The resulting limit against spontaneous fission, Z2/A∼60, is in fair agreement with the original Bohr and Wheeler value of ∼48.

1.
N.
Bohr
and
J. A.
Wheeler
, “
The mechanism of nuclear fission
,”
Phys. Rev.
56
,
426
450
(
1939
).
See also D. E. Neuenschwander, “Nuclear weapons, part 2: The mechanism of nuclear fission,” The SPS Observer 34, 10–14 (2001/2002).
2.
K. S. Krane, Introductory Nuclear Physics (Wiley, New York, 1988), pp. 482–483.
3.
A. M. Weinberg and E. P. Wigner, The Physical Theory of Neuron Chain Reactors (University of Chicago, Chicago, IL, 1958), p. 130.
4.
See, for example, R. A. Serway, C. J. Moses, and C. A. Moyer, Modern Physics, 2nd ed. (Harcourt College, Fort Worth, TX, 1997), p. 542.
5.
J. K. Tuli, Nuclear Wallet Cards, 6th ed. (National Nuclear Data Center, Brookhaven National Laboratory). Available online at 〈www.nndc.bnl.gov/wallet〉.
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