The possibility of isolating uranium-235 was technically daunting, because uranium-235 and uranium-238 are chemically identical, and vary in their mass by only the weight of three neutrons.
Muligheden for at isolere uran-235 var teknisk udfordrende, fordi uran-235 og uran-238 er kemisk identiske og kun varierer i masse med vægten af 3 neutroner.