large/12904/worldswithinworlds_v1-3_1904_librivox_64kb_mp3/worldswithinworlds_01_asimov_64kb_205
large/12904/worldswithinworlds_v1-3_1904_librivox_64kb_mp3/worldswithinworlds_01_asimov_64kb
English
12904
Physicists, however, felt uneasy at using an average as standard for they were more interested in working with individual isotopes. They preferred to set ¹⁶O at 16.0000 so that the average atomic weight of oxygen was 16.0044 and all other atomic weights rose in proportion. Atomic weights determined by this system were "physical atomic weights". PHYSICISTS HOWEVER FELT UNEASY AT USING AN AVERAGE AS STANDARD FOR THEY WERE MORE INTERESTED IN WORKING WITH INDIVIDUAL ISOTOPES THEY PREFERRED TO SET OXYGEN SIXTEEN AT SIXTEEN POINT ZERO ZERO SO THAT THE AVERAGE ATOMIC WEIGHT OF OXYGEN WAS SIXTEEN POINT ZERO ZERO FOUR FOUR AND ALL OTHER ATOMIC WEIGHTS ROSE IN PROPORTION ATOMIC WEIGHTS DETERMINED BY THIS SYSTEM WERE PHYSICAL ATOMIC WEIGHTS
xistence of a more massive isotope, ²H. This isotope has almost twice the mass of the lighter one. No other isotopes of a particular atom differ in mass by so large a factor. For that reason ²H and ¹H differ in ordinary chemical properties more than isotopes usually do and Urey therefore gave ²H the special name of “deuterium” from a Greek word meaning “second”. [Illustration: _W. F. Giauque_] In 1929 the American chemist William Francis Giauque (1895- ) found that oxygen was composed of more than 1 isotope. Its atomic weight had been set arbitrarily at 16.0000 so it was a relief that 99.76% of its atoms were ¹⁶O. However, 0.20% were ¹⁸O, and 0.04% were ¹⁷O. As you see, ¹⁶O must have a mass number of slightly less than 16.0000 and it must be the more massive isotopes ¹⁷O and ¹⁸O that pull the average up to 16.0000. Disregarding this, chemists clung to a standard atomic weight of 16.000 for oxygen as it appeared in nature, preferring not to concern themselves with the separate isotopes.  DIFFER IN ORDINARY CHEMICAL PROPERTIES MORE THAN ISOTOPES USUALLY DO AND UREE THEREFORE GAVE HYDROGEN TWO THE SPECIAL NAME OF DETERIUM FROM A GREEK WORD MEANING SECOND IN NINETEEN TWENTY NINE THE AMERICAN CHEMIST WILLIAM FRANCIS GIAE BORN EIGHTEEN NINETY FIVE FOUND THAT OXYGEN WAS COMPOSED OF MORE THAN ONE ISOTOPE ITS ATOMIC WEIGHT HAD BEEN SET ARBITRARILY AT SIXTEEN EXACTLY SO IT WAS A RELIEF THAT NINETY NINE POINT SEVEN SIX PER CENT OF ITS ATOMS WERE OXYGEN SIXTEEN HOWEVER ZERO POINT TWO ZERO PER CENT WERE OXYGEN EIGHTEEN AND ZERO POINT ZERO FOUR PER CENT WERE OXYGEN SEVENTEEN AS YOU SEE OXYGEN SIXTEEN MUST HAVE A MASS NUMBER OF SLIGHTLY LESS THAN SIXTEEN POINT ZERO ZERO AND IT MUST BE THE MORE MASSIVE ISOTOPES OXYGEN SEVENTEEN AND OXYGEN EIGHTEEN THAT PULL THE AVERAGE UP TO SIXTEEN POINT ZERO ZERO DISREGARDING THIS CHEMISTS CLUNG TO A STANDARD ATOMIC WEIGHT OF SIXTEEN POINT ZERO ZERO FOR OXYGEN AS IT APPEARED IN NATURE PREFERRING NOT TO CONCERN THEMSELVES WITH THE SEPARATE ISOTOPES
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