large/12904/worldswithinworlds_v1-3_1904_librivox_64kb_mp3/worldswithinworlds_01_asimov_64kb_54
large/12904/worldswithinworlds_v1-3_1904_librivox_64kb_mp3/worldswithinworlds_01_asimov_64kb
English
12904
The first inkling and the first proof that this might not be so came through the study of radioactivity. THE FIRST INKLING AND THE FIRST PROOF THAT THIS MIGHT NOT BE SO CAME THROUGH THE STUDY OF RADIOACTIVITY
upper end, thorium is 90 and uranium is 92. Each uranium atom has a nucleus bearing a charge of +92 and contains 92 electrons to balance this. Once the notion of the atomic number was worked out, it became possible to tell for certain whether any elements remained as yet undiscovered and, if so, where in the list they might be. Thus, when Moseley first presented scientists with the atomic number it turned out that there were still 7 elements that were not discovered. At least elements with atomic numbers of 43, 61, 72, 75, 85, 87, and 91 were still not known. By 1945, all seven had been discovered. It quickly turned out that the atomic number was more fundamental and more characteristic of a particular element than was the atomic weight. [Illustration: _Niels Bohr_] [Illustration: _Bohr’s study._] Since Dalton’s time it had been assumed that all the atoms of a particular element were of equal atomic weight and that atoms of two different elements were always of different atomic weight. D URANIUM IS NINETY TWO EACH ERRANIUM ATOM HAS A NUCLEUS BEARING A CHAR OF PLUS NINETY TWO AND CONTAINS NINETY TWO ELECTRONS TO BALANCE THIS ONCE THE NOTION OF THE ATOMIC NUMBER WAS WORKED OUT IT BECAME POSSIBLE TO TELL FOR CERTAIN WHETHER ANY ELEMENTS REMAINED AS YET UNDISCOVERED AND IF SO WHERE IN THE LIST THEY MIGHT BE THUS WHEN MOSELY FIRST PRESENTED SCIENTISTS WITH THE ATOMIC NUMBER IT TURNED OUT THAT THERE WERE STILL SEVEN ELEMENTS THAT WERE NOT DISCOVERED AT LEAST ELEMENTS WITH ATOMIC NUMBERS OF FORTY THREE SIXTY ONE SEVENTY TWO SEVENTY FIVE EIGHTY FIVE EIGHTY SEVEN AND NINETY ONE WERE STILL NOT KNOWN BY NINETEEN FORTY FIVE ALL SEVEN HAD BEEN DISCOVERED IT QUICKLY TURNED OUT THAT THE ATOMIC NUMBER WAS MORE FUNDAMENTAL AND MORE CHARACTERISTIC OF A PARTICULAR ELEMENT THAN WAS THE ATOMIC WEIGHT SINCE DALTON'S TIME IT HAD BEEN ASSUMED THAT ALL THE ATOMS OF A PARTICULAR ELEMENT WERE OF EQUAL ATOMIC WEIGHT AND THAT ATOMS OF TWO DIFFERENT ELEMENTS WERE ALWAYS OF DIFFERENT ATOMIC WEIGHT
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