large/12904/worldswithinworlds_v1-3_1904_librivox_64kb_mp3/worldswithinworlds_01_asimov_64kb_116
large/12904/worldswithinworlds_v1-3_1904_librivox_64kb_mp3/worldswithinworlds_01_asimov_64kb
English
12904
No other isotopes of a particular atom differ in mass by so large a factor. NO OTHER ISOTOPES OF A PARTICULAR ATOM DIFFER IN MASS BY SO LARGE A FACTOR
occurs in nature, is nearly a whole number and yet it is made up of more than 1 isotope. In that case, one of the isotopes makes up very nearly all of it, while the others are present in such minor quantities that the average is hardly affected. Helium, for instance (atomic symbol He) has an atomic weight of just about 4 and, indeed, almost all the atoms making it up are ⁴He. However, 0.0001% of the atoms, or one 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. RESENT IN SUCH MINOR QUANTITIES THAT THE AVERAGE IS HARDLY AFFECTED HELIUM FOR INSTANCE ATOMIC SYMBOL H E HAS AN ATOMIC WEIGHT OF JUST ABOUT FOUR AND INDEED ALMOST ALL THE ATOMS MAKING IT UP ARE HELIUM FOUR HOWEVER ONE ONE THOUSANDTH OF ONE PER CENT OF THE ATOMS OR ONE OUT OF A MILLION R HELIUM THREE AGAIN NINETY NINE POINT SIX PER CENT OF ALL THE NITROGEN ATOMS ATOMIC SYMBOL N ARE NITROGEN FOURTEEN BUT ZERO POINT FOUR PER CENT ARE NITROGEN 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
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