Brief History of The Development Of Periodic Table :
The quest for discovery of new element is ever increasing. At present, elements up to atomic number 118 are known. These elements include heavier radioactive man made elements and naturally occuring elements. Efforts to synthesize new elements are continuous process. It is difficult to study chemistry of large number of elements separately. Hence there is need number of elements separately. Hence there is need to classify these elements. Classification of elements into groups and development of periodic law and Periodic Table are the consequences of systematizing the knowledge gained by number of scientists through their observations and experiments. Prominent attempts in this field are summarized below :
1) Unitary theory :
In year 1815, William Prout suggested that the values of the atomic weights (atomic masses) of all elements were whole numbers or varied only slightly from the whole numbers, it hydrogen was considered the basic of all atomic weights.
This theory implied that all the elements were made up of hydrogen atoms.
For example 12C is made up of 12 units of hydrogen or 40Ca is made up of hydrogen. Hence atomic weight must be whole number. This theory was ruled out because it was found that copper has atomic weight 63.5 and chlorine 35.5. The existence of two isotopes of copper having atomic weights 63 and 65 was not known. Similarly existence of two isotopes of chlorine, 35, and 37 was not known in those days.
2) Dobereiner's triads :
In 1817 Dobereiner, a German chemist found that element could be arranged in a group of three called triad in such a way that middle element has an atomic weight almost the average of the other two. The three elements of a tried were found to possess similar, properties. Since Dobereiner's relationship, referred to as the law of triads, seemed to work for a few element, it was dismissed as coincidence.
|
Element |
Atomic Weight |
Element |
Atomic Weight |
|
Li Na K |
7 23 39 |
Ca Sr Ba |
40 88 137 |
|
Cl Br I |
35 80 127 |
|
|
Dobereiner's triads
3) Cooke's homologous series :
In 1854, J.P Cooke classified the elements in several homologous series on the basis of their physical and chemical properties. He found that the atomic weights of the elements present in a homologous series increase in a regular fashion.
 |
| Cooke's homologus series |
4) Newland's law of octaves :
John A.R. Newland, an English chemist, found in 1865 that if the elements known as that time were arranged in the increasing order of their atomic weights, the properties of every eighth elements were similar to those of the first one. Newland called it as law of octaves. Some of octaves formed by Newland are depicted.
The relationship was just like every eights note that resemble the first octaves of music. Newland's law of octaves seemed to the true only for elements upto calcium. The law of octaves failed in the case of elements of higher atomic weights. Moreover, the discovery of inert gases gave a big set back to this law as they were found to be quite different from other known elements and did not obey the law of octaves.
 |
| Newland's octaves |
5) Lother Meyer's arrangement of elements :
In 1869, Lother Meyer a German chemist platted various physical properties of element.
For example, atomic volume, density, melting point, boiling point, thermal conductivity etc. Were platted against atomic weights and found that these properties varied in periodic fashion. On the basis of this study, he arranged elements in such a way that, it resembled nearly to Mendeleev's arrangement of elements.
However, Lother Meyer is more remembered for his atomic volume curve than for his arrangement of elements.
 |
| Mendeleev's periodic table published earlier |
6) Mendeleev's periodic law and its significance :
The first meaningful and remarkable contribution in the field of classification of elements was made by a Russian chemist, Dimitri Mendeleev in 1869. Mendeleev used a broader range of physical and chemical properties. of elements as compared to those used by Lother Meyer and studied the formulae and properties of several compounds of element. On the basis of these studies, he developed a law known as Mendeleev's periodic law. This law can be stated as follows.
The physical and chemical properties of elements are the periodic function of their atomic weights (atomic masses). Mendeleev arranged elements in horizontal rows and vertical columns of a table in order of their increasing atomic masses in such as way that the elements with similar properties occupied the same vertical column or group. He relied on the similarities in the empirical formulae and properties of the compounds formed by the elements. He realized that some of the elements did not fit in with his scheme of classification if the order of atomic mass was strictly followed.
He ignored the order of atomic masses, thinking that atomic mass measurements might be incorrect, and placed the elements with similar properties together. For example, iodine with similar properties together. For example, iodine with lower atomic weight than that of tellurium was placed in group VII along with fluorine, chlorine, bromine because of the similarities in properties.
At the sometime, keeping the primary aim of arranging the elements of similar properties in the same group, he proposed that some of the elements were still undiscovered and, therefore, left several gaps in the periodic table. For example, both gallium and germanium were unknown at the time when Mendeleev published periodic table. He left the gap one place down aluminium and Eka-silicon which were discovered afterwards. Mendeleev predicted not only the existence of gallium and germanium, nut also described some of their general physical properties. The boldness of Mendeleev's quantitative predictions and their eventual success made him and his Periodic table famous. Mendeleev's Periodic Table was published in 1905.