Atomic number and the mass defect of nucleon binding is always small compared to the nucleon mass, the atomic mass of any atom, when expressed in unified atomic mass units (making a quantity called the "relative isotopic mass"), is within 1% of the whole number "A". Atoms with the same atomic number "Z" but different neutron numbers "N", and hence different atomic masses, are known as isotopes. A little more than three-quarters of naturally occurring elements exist as a mixture of isotopes (see monoisotopic elements), and the average isotopic mass of an isotopic mixture for an element (called the relative atomic mass)
Otto Hahn pupil and assistant Fritz Strassmann (1902–1980), Otto Hahn furthered the research begun by Enrico Fermi and his team in 1934 when they bombarded uranium with neutrons. Until 1938, it was believed that the elements with atomic numbers greater than 92 (known as transuranium elements) arose when uranium atoms were bombarded with neutrons. The German chemist Ida Noddack proposed an exception. She anticipated the paradigm shift of 1938/39 in her article published in the journal "Angewandte Chemie", Nr. 47, 1934, in which she speculated: It is conceivable that when heavy nuclei are bombarded with neutrons these nuclei could break down into