large/12904/worldswithinworlds_v1-3_1904_librivox_64kb_mp3/worldswithinworlds_01_asimov_64kb_189
large/12904/worldswithinworlds_v1-3_1904_librivox_64kb_mp3/worldswithinworlds_01_asimov_64kb
English
12904
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. 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
ame to be called “mass spectrometers”. The first to use the name was the English physicist Francis William Aston (1877-1945), who built the first efficient instrument of this type in 1919. He used it to study as many elements as he could. He and those who followed him located many isotopes and determined the frequency of their occurrence with considerable precision. It turned out, for instance, that neon is actually 90.9% ²⁰Ne, and 8.8% ²²Ne. Very small quantities of still a third 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.  AS MANY ELEMENTS AS HE COULD HE AND THOSE WHO FOLLOWED HIM LOCATED MANY ISOTOPES AND DETERMINED THE FREQUENCY OF THEIR OCCURRENCE WITH CONSIDERABLE PRECISION IT TURNED OUT FOR INSTANCE THAT NEEON IS ACTUALLY NINETY POINT NINE PER CENT NEON TWENTY AND EIGHT POINT EIGHT PER CENT NEON TWENTY TWO VERY SMALL QUANTITIES OF STILL A THIRD ISOTOPE NION TWENTY ONE ARE ALSO PRESENT MAKING UP THREE TENTHS OF ONE 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
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