Classification became necessary due to the increase in the number of elements discovered. In 1817, J.W. Dobereiner, a German chemist , explained that the atomic weight of strontium rests in the middle between that of calcium and barium, and few years later he demonstrated that other such “triads” such as chlorine, bromine, and iodine and lithium, sodium, and potassium are also present. Other scientists, during1827 and 1858 developed analogous relationships which extended more than the triads of elements, fluorine being added to the halogens and magnesium to the alkaline-earth metals, while oxygen, sulfur, selenium, and tellurium were classed as one family and nitrogen, phosphorus, arsenic, antimony, and bismuth as one more element family. De Chancourtois, a French scientist, in 1862 anticipated a classification of the elements based on the new values of atomic weights. It was finally the Russian chemist, Mendeleev, who proposed the periodic law, which indicated that the elements arranged in the increasing order of atomic weights show a periodic change of properties. In 1869, the first periodictable was tabulated by Mendeleev. It had 17 columns, with two nearly complete periods of elements, frompotassium to bromine and rubidium to iodine, lead by two fractional periods of seven elements each (lithium to fluorine and sodium to chlorine), and three incomplete periods.
What is the importance of the Periodic law?
- Innovation of NewElements: Through Mendeleyev’s efforts in 1871, the grand significance of the periodic law was made apparent in predicting that the properties of the 17elementscould be interrelated with those of otherelementsby relocating the 17 to new positions from those shown by their atomic weights. The subsistence of many of the properties of uninventedelementsof that time like eka-boron, eka-aluminum, and eka-silicon, now identified with theelementsscandium, gallium, and germanium, was also predicted by Mendeleev. Likewise, following the discovery of helium and argon, the periodic law allowed the discovery of the subsistence of neon, krypton, xenon, and radon. Besides, the absence ofelement72 was anticipated, from its position in the periodic system, to be alike to zirconium in its properties rather than to the rare earths, was shown byNielsBohr, a Danish physicist. In 1922, other scientists also examined zirconium ores following Bohr’s prediction.
- Implication of atomic numbers: A number of of the elements in the Mendeleyev periodic tables were necessary to arrange elements according to their atomic weight . For example, in the pair’s of argon and potassium, cobalt and nickel, and tellurium and iodine, the first element had the previous place in the periodic system but with a bigger atomic weight. When the structure of the atom was studied, the explanation to this complexity was solved. Research done by Ernest Rutherford on the scattering of alpha particles by the nuclei of heavy atoms, in1910, helped the prediction of the nuclear electrical charge. Approximately, the ratio of the nuclear charge to that of the electron was noted to be one-half the atomic weight. Another scientist, in 1911 recommended that this quantity, the atomic number, might be recognized with the ordinal number of the element in the periodic table. In 1913, this proposal was vividly established by the English physicist, H.G.J. Moseley’s measurements of the wavelengths of the characteristicX-rayspectrallinesof manyelements, which showed that the wavelengths relied in a usual way on the atomic numbers indistinguishable with the ordinal numbers of theperiodictableelements.
How did the periodic law evolve ?
Starting in 1913, thorough knowledge of the the elements and their properties had improved. Followed by the exclusion principle by the Austrian theoretical physicist, Wolfgang Pauli in 1925, the invention of the spin of the electron by George E. Uhlenbeck and Samuel Goudsmit in 1925, and the development of quantum mechanics by Werner Heisenberg were the significant advanced inventions which developed the periodic law. The development of the electronic theory of valence and molecular structure, beginning with the postulate of the shared electron pair by the chemist, Gilbert N. Lewis in 1916, also played a vital role in elucidating the periodic law. 
The Periodic Table
The Periodic Table
"If all the elements are arranged in the order of their atomic weights, a periodic repetition of properties is obtained. This is expressed by the law of periodicity."
The periodic table we use today is based on the one devised and published by Dmitri Mendeleev in 1869.
Mendeleev found he could arrange the 65 elements then known in a grid or table so that each element had:
1. A higher atomic weight than the one on its left. For example, magnesium (atomic weight 24.3) is placed to the right of sodium (atomic weight 23.0):
2. Similar chemical properties to other elements in the same column - in other words similar chemical reactions. Magnesium, for example, is placed in the alkali earths' column:
Mendeleev realized that the table in front of him lay at the very heart of chemistry. And more than that, Mendeleev saw that his table was incomplete - there were spaces where elements should be, but no-one had discovered them.
Just as Adams and Le Verrier could be said to have discovered the planet Neptune on paper, Mendeleev could be said to have discovered germanium on paper. He called this new element eka-silicon, after observing a gap in the periodic table between silicon and tin:
Similarly, Mendeleev discovered gallium (eka-aluminum) and scandium (eka-boron) on paper, because he predicted their existence and their properties before their actual discoveries.
Although Mendeleev had made a crucial breakthrough, he made little further progress. With the benefit of hindsight, we know that Mendeleev's periodic table was underpinned by false reasoning. Mendeleev believed, incorrectly, that chemical properties were determined by atomic weight. Of course, this was perfectly reasonable when we consider scientific knowledge in 1869.
In 1869 the electron itself had not been discovered - that happened 27 years later, in 1896.
In fact, it took 44 years for the correct explanation of the regular patterns in Mendeleev's periodic table to be found.