ISAAC ASIMOV Boston University School of Boston, Massachusetts
T H E constituents of the dry atmosphere are given in most textbooks of chemistry in a form similar to that shoum in Table 1. I n one respect, such a table is no longer entirely adequate. To he sure, it includes the only four gases which are present in the dry atmosphere in more than trace amounts. However, to the knowledge of the author, such tables never distinguish among the various isotopic species of the gases concerned. The interest in a table which would so distinguish lies in the fact that the heavy isotopes of oxygen and nitrogen are useful in chemical and biochemical investigations. The atmosphere is one source of heavy oxygen and is the sole significant source of heavy nitrogen.
Among the still rarer constituents of the atmosphere are the inert gases. Except for the helium occurring along with natural gas in certain wells, the atmosphere is the only significant source of these elements. The TABLE 3 Composition of the Dry Atmosphere by Nuclides Nuclides per len lho~isawd nuclides N" .A4O
N1" 0'6
0" C" A36 A11 others
Pads per ten thousand ( b y volume)
TABLE 4 Inert-gas Composition of the Dry Atmosphere
7809 2095 93 3 Traces
Parts billion (by per volume)
Such an important chemical resource is worthy of quantitative estimation. Using the per cent isotopic composition of the elements concerned (which may be found in any handbook) and assuming random distribution of the various nuclides, it is possible to prepare Table 2, which takes isotopic composition into account. It is also possible to center attention not on molecules hut on the nuclides themselves and to prepare Table 3. It is obvious from Tables 2 and 3 that the heavy isotopes of nitrogen and oxygen, while only minor constituents of the atmosphere, are far from being trace substances.
Argon Neon Helium Krypton Xenon
TABLE S Insrtqes Composition of the Dry Atmosphere by Nuclides
Parts per ten thousand ( b y volume) 0'60L'
A4a N"NL6 01Q018
,pO'QL' OIQ" All others
9,300,000 18,000 5000 1000 90
-
TABLE 2 Composition of the Dry Atmosphere by Molecular Species
N14N14
7818 2101 46.5 28.7 4.3 0.8 0.5 0.2 Traces
0'6
TABLE 1 Composition of the Dry Atmosphere
Xitrogen Oxygen Argon Carbon dioxide All others
Medicine,
Ports per billion (by volume) A'o A' Ne2O AJ8 He'
Nos'
Kre4 Kra' Kra' KrSa Nezl
Se13' Xe'29 Krm Xelal Xe'a' );e1ae XeIm Kr'* XelP8 XdZ4 Xe"' He'
7753 2085 93 56 8 3 1.6 0.4 Traces
633
9,260,000 32,000 16,300 5600 5000 1700 560 170 120 120 50 24 23 20 19 9 8 3
3 1.8 0.09 0.08 0.006
634
concentration of these gases in the atmosphere is given in Table 4, argon being included for the sake of completeness.
JOURNAL OF CHEMICAL EDUCATION
billion cubic miles. I t follows that a figure representing the volume of an atmospheric constituent as parts per billion also represents the volume in cubic miles made up by that constituent if it were collected in one place under those standard conditions of temperature and TABLE 6 Rare Molecular Species of Oxygen and Nitrogen in the pressure. Atmosphere Thus, there is enough Xe128in the air, trace constituent though it be, to make up nearly two cubic miles of Parts pe7 billion (by volume) gas if it were collected in one spot. Even He3, the least of the inert-gas nuclides, both in mass and in frequency of occurrence, would take up 0.006 cubic miles if collected which amounts t o 24 billion liters or, in mass, 3.2 billion grams. I n fact, even though He3 occurs in the atmosphere only to the extent of six parts The five stable inert gases listed in Table 4 are made in a trillion, there is still enough of it for every cubic up of a total of 23 different stable nuclides. Knowing centimeter of air (at standard conditions) to contain the isotopic composition of the inert gases, Table 5 160,000,000 atoms of that nuclide. One last note: There are species of oxygen and nitromay be prepared in which the concentration of each nuclide is given. gen molecules whose occurrence in the atmosphere is The unit, parts per billion, was selected for use in of the order of rarity of the inert gas nuclides. These, Tables 4 and 5 because the volume of the atmosphere, naturdy, are the molecules made up entirely of the calculated to 0°C. and one atmosphere pressure, is one rarer isotopes of these elements (see Table 6).
