The Making of the Atomic Bomb: 25th Anniversary Edition

by Richard Rhodes

Frisch: “I remember that I immediately at that instant thought of the fact that electric charge diminishes surface tension.”988 The liquid drop is held together by surface tension, the nucleus by the analogous strong force. But the electrical repulsion of the protons in the nucleus works against the strong force, and the heavier the element, the more intense the repulsion. Frisch continues: And so I promptly started to work out by how much the surface tension of a nucleus would be reduced. I don’t know where we got all our numbers from, but I think I must have had a certain feeling for the binding energies and could make an estimate of the surface tension. Of course we knew the charge and the size reasonably well. And so, as an order of magnitude, the result was that at a charge [i.e., an atomic number] of approximately 100 the surface tension of the nucleus disappears; and therefore uranium at 92 must be pretty close to that instability. They had discovered the reason no elements beyond uranium exist naturally in the world: the two forces working against each other in the nucleus eventually cancel each other out.