large/12904/worldswithinworlds_v1-3_1904_librivox_64kb_mp3/worldswithinworlds_01_asimov_64kb_242
large/12904/worldswithinworlds_v1-3_1904_librivox_64kb_mp3/worldswithinworlds_01_asimov_64kb
English
12904
Its atomic weight had been set arbitrarily at 16.0000 so it was a relief that 99. ITS ATOMIC WEIGHT HAD BEEN SET ARBITRARILY AT SIXTEEN EXACTLY SO IT WAS A RELIEF THAT NI
e out of a million, are ³He. Again, 99.6% of all the nitrogen atoms (atomic symbol N) are ¹⁴N, but 0.4% are ¹⁵N. Then, 98.9% of all carbon atoms (atomic symbol C) are ¹²C, but 1.1% are ¹³C. It is not surprising that the atomic weights of nitrogen and carbon are just about 14 and 12, respectively. [Illustration: _Harold Urey_] Even hydrogen does not escape. Its atomic weight is just about 1 and most of its atoms are ¹H. The American chemist Harold Clayton Urey (1893- ) detected the existence 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. ITROGEN FIFTEEN THEN NINETY EIGHT POINT NINE PER CENT OF ALL CARBON ATOMS ATOMIC SYMBOL C ARE CARBON TWELVE BUT ONE POINT ONE PER CENT ARE CARBON THIRTEEN IT IS NOT SURPRISING THAT THE ATOMIC WEIGHTS OF NITROGEN AND CARBON ARE JUST ABOUT FOURTEEN AND TWELVE RESPECTIVELY EVEN HYDROGEN DOES NOT ESCAPE ITS ATOMIC WEIGHT IS JUST ABOUT ONE AND MOST OF ITS ATOMS ARE HYDROGEN ONE THE AMERICAN CHEMIST HAROLD CLAYTON UREE BORN EIGHTEEN NINETY THREE DETECTED THE EXISTENCE OF A MORE MASSIVE ISOTOPE HYDROGEN TWO 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 HYDROGEN TWO AND HYDROGEN ONE 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
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