large/12904/worldswithinworlds_v1-3_1904_librivox_64kb_mp3/worldswithinworlds_01_asimov_64kb_237
large/12904/worldswithinworlds_v1-3_1904_librivox_64kb_mp3/worldswithinworlds_01_asimov_64kb
English
12904
The radioactive elements did not escape this new view either. The atomic weight of uranium (chemical symbol U) is just about 238 and, indeed, most of its atoms are ²³⁸U. THE RADIOACTIVE ELEMENTS DID NOT ESCAPE THIS NEW VIEW EITHER THE ATOMIC WEIGHT OF URANIUM CHEMICAL SYMBOL U IS JUST ABOUT TWO THIRTY EIGHT AND INDEED MOST OF ITS ATOMS ARE URAN
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. 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”. Finally, in 1961, a compromise was struck. Chemists and physicists alike decided to consider the atomic weight of ¹²C as exactly 12 and to use that as a standard. By this system, the atomic weight of oxygen became 15.9994, which is only very slightly less than 16. SIVE 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 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 FINALLY IN NINETEEN SIXTY ONE A COMPROMISE WAS STRUCK CHEMISTS AND PHYSICISTS ALIKE DECIDED TO CONSIDER THE ATOMIC WEIGHT OF CARBON TWELVE AS EXACTLY TWELVE AND TO USE THAT AS A STANDARD BY THIS SYSTEM THE ATOMIC WEIGHT OF OXYGEN BECAME FIFTEEN POINT NINE NINE NINE FOUR WHICH IS ONLY VERY SLIGHTLY LESS THAN SIXTEEN
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