So far we have been looking at discoveries and some attempt at understanding events. This section on the periodic table initially takes us back to the 18th century and brings us quickly into the 19th century and then the 20th century with a new outlook. No longer would scientists be making "chance" chance observations but would predict and prove based on established ideas. When that didn't work they would rethink their predictions and reprove them.
The story starts, with the English chemists Smithson Tennant (1761-1815), Charles Hatchett; together with Swedish chemist, Anders Gustaf Ekeberg (1767 - 1813); and several others rapidly discovering new elements. By the year 1830, fifty-five different elements were recognized and was growing. IT was becoming difficult for chemists to work with. How many more were there, would it ever end. It seemed that there was an endless supply of elements to be discovered and no logical way of cataloguing them.
Rapidly, chemists began scrambling for a way to give order to this ever growing list of elements. The first sign of of reprieve came from German chemist Johann Wolfgang Döbereiner (1780 -1849). In 1829, Dobereiner noted that the element bromine seemed to have properties that were about halfway between chlorine and iodine. He went on to find a similar pattern for calcium, strontium and barium; and for sulfur, selenium and tellurium. He named these groups "triads" but was unsuccessful in finding any more and many scientist felt that these triads were coincidence.
The growing concern over organization of the elements and their different weights led the German chemist, Friedrich August Kekulé von Stradonitz (1829 -86) who was already well known for his structural work,to suggest an international scientific meeting to resolve the problems. In 1860, in the town of Karlsruhe, Germany, the First International Chemical Congress was held. It was attended by one hundred and forty scientists from around the world. It was the Italian chemist Stanislao Cannizzaro (1826 - 1910) who saw how his countryman, Avogadro's, ideas could be used here to resolve a lot of the problems and distinguish between the different numbers being used. He was also keenly aware of the importance of not confusing atomic and equivalent weight. At the congress, slowly and carefully won over the scientific world to his ideas. From then on, the matter of atomic weights was clarified and the importance of Berzelius's table of atomic weights appreciated.
The next major step was in 1864 when the English chemist John Alexander Reina Newlands (1837 - 98) arranged the known elements in order of increasing atomic weights, and noted that there was also some partial order of their properties. When he made colums of 7 elements, some of the rows fell into the triads described by Döberiener. He called this the "Law of Octaves". Unfortunately, many still thought this was coincidence and he was unable to publish his work. In 1870, the German chemist, Julius Lothar Meyer (1830 - 95) was a little more successful in getting his work published. Mayer considered the volume taken by a certain fixed weight of the various elements and believed proposed that under certain conditions, each weight contained the same numbers of its particular type. This mean that the ratio of volumes of various elements was equal to the ratio of the volumes of single atoms of the various elements. Therefore one could speak of them as "atomic volumes". When these volumes were plotted against the weights a eries of waves developed that rose to sharp peaks at the metals, lithium, sodium, potassium, rubidium and cesium. and each rise and fall corresponded to a period in a table of elements. It even showed where Newlands had made a mistake by assuming that all waves contained 8 elements. The Law of Octaves only holds for the first two periods.
However, his work was in vein. The year before, Russian chemist Dmitri Ivanovich Mendeléev (1837 - 1907) had also discovered the change in length of the periods of elements, and dramatically demonstrated its consequences. Mendeléev approached the problem from the valence stance described by the English Chemist Edward Frankland (1825 - 99) in 1852. He noticed that the earlier elements in the list showed an increase in valence,and that the valence rose and fell, establishing periods. First hydrogen, alone; then two periods of seven followed by periods containing more than seven elements. Mendeléev used this information not only to prepare a graph comparable to that of Meyer but a table comparable to that of Newlands. He avoided Newlands mistakes by not insisting on columns of seven elements. He published his table in 1869, one year before Meyer.
However, it is not just the timing that gives Mendeléev the majority of credit. In order to fit elements to the required valence he was forced to put some elements out of order on a weight basis, so tellurium (AW 127.5 Valence 2) was put ahead of Iodine (AW 126.9, Valence 1). He also found it necessary to leave gaps in his table, stating that they represented undiscovered elements and was later able to predict properties of some of these elements and named them eka-aluminum, eka-boron and eka-silicon.
Again, the scientific world remained skeptical until in 1875 the French chemist Paul Emile Lecoq de Boisbaudran (1838 -1912) dramatically found a new element (gallium, named after Gaul) that exactly matched all properties described by Mendeléev as being eka-aluminum. In 1879, the Swedish chemist, Lars Fredrick Nilson (1840 - 99) discovered Scandium (for Scandinavia) and was immediately recognized as the eka-boron from Mendeléev's table. Finally, in 1886 the German chemist, Clemens Alexander Winkler (1838 -1904) discovered germanium (after Germany) and this exactly matched Mendeléev's eka-silicon.
After this, no one could doubt the validity and usefulness of the periodic table.