large/12904/worldswithinworlds_v1-3_1904_librivox_64kb_mp3/worldswithinworlds_01_asimov_64kb_124
large/12904/worldswithinworlds_v1-3_1904_librivox_64kb_mp3/worldswithinworlds_01_asimov_64kb
English
12904
The integer closest to the mass of the individual isotope is spoken of as the "mass number" of that isotope. THE ENTER CLOSEST TO THE MASS OF THE INDIVIDUAL ISOTOPE IS SPOKEN OF AS THE MASS NUMBER OF THAT ISOTOPE
 ²¹Ne, are also present, making up 0.3%. As for ordinary lead in nonradioactive rocks, it is made up of 23.6% ²⁰⁶Pb, 22.6% ²⁰⁷Pb, and 52.3% ²⁰⁸Pb. There is still a fourth isotope, ²⁰⁴Pb, which makes up the remaining 1.5% and which is not the product of any radioactive series at all. The isotopes always have atomic weights that are close to, but not quite, whole numbers. Any atomic weight of an element that departs appreciably from an integer does so only because it is an average of different isotopes. For instance, the atomic weight of chlorine (chemical symbol Cl) is 35.5, but this is because it is made up of a mixture of 2 isotopes. About one quarter of chlorine’s atoms are ³⁷Cl and about three-quarters are ³⁵Cl. [Illustration: _Francis W. Aston_] [Illustration: _Mass spectrograph as used by Thomson and Aston to measure the atomic weight of neon._] To avoid confusion, the average mass of the isotopes that make up a particular element is still called the atomic weight of that element. NE PER CENT AS FOR ORDINARY LEAD IN NON RADIOACTIVE ROCKS IT IS MADE UP OF TWENTY THREE POINT SIX PER CENT LEAD TO O SIX TWENTY TWO POINT SIX PER CENT LEAD TO O SEVEN AND FIFTY TWO POINT THREE PER CENT LEAD TO O EIGHT THERE IS STILL A FOURTH ISOTOPE LEAD TO O FOUR WHICH MAKES UP THE REMAINING ONE POINT FIVE PER CENT AND WHICH IS NOT THE PRODUCT OF ANY RADIOACTIVE SERIES AT ALL THE ISOTOPES ALWAYS HAVE ATOMIC WEIGHTS THAT ARE CLOSE TO BUT NOT QUITE WHOLE NUMBERS ANY ATOMIC WEIGHT OF AN ELEMENT THAT DEPARTS APPRECIABLY FROM AN ENTAGER DOES SO ONLY BECAUSE IT IS AN AVERAGE OF DIFFERENT ISOTOPES FOR INSTANCE THE ATOMIC WEIGHT OF CHLORINE CHEMICAL SYMBOL C L IS THIRTY FIVE POINT FIVE BUT THIS IS BECAUSE IT IS MADE UP OF A MIXTURE OF TWO ISOTOPES ABOUT ONE QUARTER OF CHLORANE'S ATOMS ARE CHLORING THIRTY SEVEN AND ABOUT THREE QUARTERS ARE CHLORINE THIRTY FIVE TO AVOID CONFUSION THE AVERAGE MASS OF THE ISOTOPES THAT MAKE UP A PARTICULAR ELEMENT IS STILL CALLED THE ATOMIC WEIGHT OF THAT ELEMENT
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