The Periodic System
Classification based on Atomic Weight
According to the present point of view the atomic number is a more fundamental characteristic of an atom than its atomic weight. A value for the latter ism with a few exceptions, merely an average of the atomic masses of the isotopes of an element, and may be altered by various processes used to change isotopic ratios. However, an atomic number is an inalterable value and thus it represents a really fundamental property of an atom.
1. Döbereiner's Triads
In 1829, Döbereiner called attention for the first time to the suggestive relation between the atomic weights and the properties of elements. He pointed out that, from the elements, sets of three similar elements could be chosen having atomic weights such that the atomic weight of the central member is the mean of the other two.
Such groups of three elements he called triads. Example of this are:
Where the arithmetical mean of the atomic weights of chlorine and iodine is 81,19
2. Classification of DeChancourtois
This French engineer and geologist may in some measure be regarded as the first to devise a comprehensive scheme of classification based on atomic weights. His work was rather obscure and contained a number of arbitrary assumptions which prevented its general acceptance. However, the idea of periodicity was clearly indicated in this scheme.
3. Law of Octaves
In 1864 Newlands observed that if the known elements (beginning with lithium, for example) are arranged in the order of their atomic weights, the eighth element (in this case sodium) has properties closely related to the first. Of this relation, which he termed the law of octaves, he said; "The eighth element starting from a given one is a kind of repetition of the first, like the eighth note of an octave of music". Newlands was ridiculed in a meeting of the Chemical Society of London. Later however, the importance of the law was recognized.