F. H. Firsching School of Science 8 Technology, Southern Illinois University, Edwardsville, IL 62026 Annarent anomalies in which the atomic mass sequence of the riements dues not follow the atomic number &equewe occur three times in the Periodic Table of the Eltments (see Tahle 1.)Satisfactory explanation of these departures from a regular sequence are not uiually giwn in text bwks. All the information corltuined in the I'eriodic'l'ahle of the Elements has k e determilled ~ ~ from earth materiills, and this must be considered in examining the anomalies. The abundances of the elements throughout the universe are not identical to thiabundances of the elements present in the earth. This has important significance in considering the first anomalv. the reversal of the atomic mass seauence of areon and pot"ak4um. If the composition of the ~ i i k Way y ~alaxy or the entire universe is considered. then the relative atomic masses for argon and potassium as given in the Periodic Table of the Elements are not correct. The origin of the elements has been studied thoroughly ( I , 2) and shows that the alpha process, the addition of a helium nucleus into the nucleus of another element, is a common mode of element synthesis. For this reason, argon-36, composed of nine alpha particles, would be expected to be very abundant. And it is in the universe, but not on the earth. During the formation of the earth from a cloud of gas and dust, some severe fractionation occurred. Volatile elements tended to escape the condensation process and practically all of the argon was lost. To be more specific, the argon-36, the most abundant argon isolqw in the universe, esaped almost completely. The argon-36 retained hp the earth has been estimated ro be only about :I X 10-q < the,i original quantity present in the gas cloud inatmial from which the earth condensed (31. twimitive earth had onlv . . Co~wauentlv. ..th~.earlv. ".. a relatively minute amount of argon present. Nevertheless, the atmosphere of the earth today contains 0.934% by volume of argon ( 4 ) , and the relative abundance of the various isotopes is S 6 h = 0.337%. %r = 0.0ji3%, = 99.600% (5). Practically all of the argon found in the world todav is areon-40. The way this situation came about involves the radioactive decay of potassium-40. As the earth was forming, the volatile elements tended to be lost. However, potassium, a constituent of rocks, tended to be retained. About 2.40% of the modern earth's crust is potassium (6).The radioactive isotope, potassium-40, is about 0.118% of the total (5). Potassium-40 has two methods of decay; about 89% decays to form calcium-40, while 11%decays to form argon-40 (5). ~
~
~
~
-
+ 0-
-
4 0 ~ 40c.
4 0 +~ e"Ar The half-life of potassiu~-40is 1.26 X 109years (5).With the age of the earth a t 4.6 X lo9years ( 3 ) ,there has been ample time for most of the original notassium-40, present in the primitive earth, to undergo a n k l e a r transformation, Calculations indicate that the decay of potassium-40 can account for the amount of argon-40 in today's atmosphere. The argon-40 produced in this way escaped from the interior of the earth by volcanic action throughout the long history of the earth.
Presented at the 15th Midwest Regional Mwting of the American Chemical Society. St. Louis, MO,Nov. 9, 1979. 478
Journal of Chemical Education
Table 1. Three Atomic Mass Anomalies Atomic Number
Ammic Mass
Argon
18
Pota~sIum Cobalt Nickel
19 27
39.948 39.098 58.9332 58.70
Element
Tellurium Iodine
28 52
127.60
53
126.9045
The result is that the argon found in the earth's atmosphere today is skewed drastically in favor of one isotope, argon-40. The isotopic argon distribution found on earth is not representative of the cosmic abundance. T h e anomaly of argon having an atomic mass higher than potassium is Gue for the earth hut not the cosmos. The other two anomalies, cobalt-nickel and telluriumiodine, are due to the inherent nuclear stability of even atomic numbered elements as compared to odd atomic numbered elements. (Hydrogen is the exception.) About 86% of all the nuclides in the earth's crust have even mass numbers (also even atomic numbers), while about 14% have odd mass numbers (also odd atomic numbers) (5,7). Within the Periodic Table of the Elements there are twenty-one elements that are mono-isotopic; 9Be, I9F, 23Na, 27A1,S I P , 45Sc, 55Mn, 59Co, 75As. 89Y, gaNh, lo3Rh, lZ75 1B3Cs, l4lPr. 15'Tb, '65Ho, l ' T m , Ig7Au,209Bi,and 232Th(5). 1\11of the
