Of the many attempts to base classifications of the elements upon unchanging constants of the atoms the first of any great value was that of Dobereiner in 1829. He called attention to the fact that it is possible to select elements in groups of three, the chemical behavior of the elements being similar, but also progressing relatively with increase of atomic weight in such a manner that the atomic weight of the central member is approximately the mean of the other two, while the properties of the second member are intermediate between the first and third. He called such groups of elements triads. Although this relalion was fragmentary, it was the forerunner of our present periodic system and it is interesting to note that each set of the triads of Dobereiner is now found in its appropriate group of the periodic table. Just as the discovery of atomic numbers made the periodic classification more useful and interesting, the working out of the idea of Dobereiner with atomic numbers, instead of atomic weights, enables us not only to get an exact, instead of an approximate, relationship but indicates the underlying cause. The relation will be clearly seen by reference to the table in which are listed some of the triads, with both atomic weights and atomic numbers, it being observed that the relationship is approximate only with the atomic weights, but is exact with the atomic numbers. ~leneni
Chlorine Bromine Iodine Sulfur Selenium Tellurium Calcium Strontium Barium Phosphorus Arsenic Antimony
lromic Wcigkr
Mcon
35.457 79.916 126.932 32.064 79.2 127.5 40.07 87.63 157.37 31.027 74.96 121.77
81.19
79.78
88.72
76.40
~ ~ o m~i ucm b r r .wean
17 35 53 16 34 52 20 38 56 15 33 51
35
34
38
33
The arithmetic explanation of the exact relationship shown by the atomic numbers will be obvious to one who notes that, on a periodic chart containing the atomic numbers, a set of triads falling into the same group results in a progression of atomic number from member to member of the triad of 18 in each case. Starting with any number, adding 18 to get a second number, and adding another 18 to get a third number, the second number will be the mean of the other two. 162
