TRANSURANIUM ELEMENTS—SEABORG 259 
making predictions of the properties for undiscovered isotopes of this 
type more certain. The rates of decay by alpha-particle emission 
and by spontaneous fission are slower for isotopes having an odd 
number of protons or an odd number of neutrons or an odd number 
of both protons and neutrons. Unfortunately for the prospect of 
producing ever higher elements, the predictions suggest shorter half- 
lives as atomic number increases. By the time elements 104 and 105 
are reached, we shall probably find that the longest-lived isotopes that 
can be made will exist barely long enough for chemical identification. 
In the case of element 104, the predicted half-lives of the longest-lived 
isotopes are measured in seconds or minutes and for element 105 in 
seconds. It should be mentioned, however, that any of these nuclides 
can have a specially hindered decay, leading to longer half-lives than 
those predicted. It is likely that the present basic criterion for the 
discovery of a new element, namely chemical identification and separa- 
tion from all previously known elements, will have to be changed at 
some point. Careful measurements of decay properties and produc- 
tion yields and mechanisms, and the clever use of recoil techniques, 
should eventually allow the extension of effective identification to 
another half-dozen elements or so beyond the heaviest now known. 
In fact the identification of the first isotopes of al] the new elements 
that will be discovered in the future probably will be accomplished 
through the use of such methods, and the production of isotopes with 
sufficiently long half-lives to allow chemical identification will follow 
later. For the isotopes with very short half-lives, some chemical 
identification can probably be made using simpler and faster methods 
involving migration, volatility, reactions with surfaces, or gas-flow 
reactions. 
Some interesting predictions concerning superheavy nuclei have 
been made. J. A. Wheeler has been able to show that extranuclear 
electrons for atoms with atomic number substantially higher than 137 
(often considered the upper limit) would behave normally because of 
the finite extension of the nucleus. Accordingly it would appear that 
there is no limitation on the existence of such heavy elements from the 
standpoint of the electronic structure of such atoms. The production 
of such nuclei would require extremely high neutron fluxes, of the 
order of 10** neutrons per square centimeter per second, such as may 
be present in stars. It is difficult to see how such nuclei can be made 
on earth. There is no indication that such superheavy nuclei can 
be produced and detected, because the rate of decay increases rapidly 
as the atomic number increases. Unless unexpected islands of sta- 
bility due to closed neutron or proton shells are found, predictions 
based on regularities in decay properties suggest that it should not be 
possible to produce and detect elements beyond another half dozen 
or so, 
