Figure 2-5. World's oldest classroom periodic table, printed between 1879 and 1886 (source).
There are three examples of an element with a higher atomic mass placed ahead of its neighbor with a lower atomic mass in the table. These are:
1. argon (Ar), #18, atomic mass = 39.95 and potassium (K), #19, atomic mass = 39.10;
2. cobalt (Co), #27, atomic mass = 58.93 and nickel (Ni), #28, atomic mass = 58.69; and
3. tellurium (Te), #52, atomic mass = 127.60 and iodine (I), #53, atomic mass = 126.90.
At the time of Mendeleev's work, argon was not discovered yet and the accuracy of atomic weight determination was not high enough to tell the rather subtle difference between those of cobalt and nickel. As for the tellurium - iodine pair, Mendeleev was aware of the discrepancy, as is witnessed by the quotation mark he put next to the atomic weight of Te in his manuscript (Figure 2-1). Nevertheless, he placed the two elements in the correct order in his table, because tellurium displayed chemical properties similar to those of selenium, sulfur, and oxygen, whereas iodine was akin to bromine, chlorine, and fluorine.
To uncover the reason behind the three exceptions (Ar-K, Co-Ni, and Te-I) and understand the true origin of the periodic law, let us move on to the next section. 2.1.3. Exercises.
1. Phrase the periodic law in your own words. Provide examples of similarities displayed by the elements constituting (a) Group 1 (the alkali metals); (b) Group 17 (the halogens); and (c) Group 18 (the noble gases) of the periodic table.
2. What was the tellurium - iodine discrepancy that Mendeleev observed when organizing the elements in the order of their atomic weights? How did he resolve this inconsistency?
3. What do the potassium-argon, cobalt-nickel, and tellurium-iodine pairs of elements tell us with regard to the periodic law as worded by Mendeleev?