Genesis of periodic classification

1.Unitary Theory

Unitary Theory a system of views in 19th-century chemistry based on the concept that the molecule is a unified whole made up of atoms of the chemical elements.In addition, the theory made possible correct values for many elements and corrections of formulas for the elements' compounds.

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, if hydrogen was considered the basis of all atomic weights. This theory implied that all the elements were made up of hydrogen atoms.

For example y, 12C is made up of 12 units of hydrogen or 40Ca is made up of 40 units 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. Döbereiner's Triads

In 1817 Dobereiner, a German chemist found that elements could be arranged in a group of three called triad in such a way that middle element had an atomic Weight almost the average of the other two. The three elements of a triad were found to possess similar, properties. Since Dobereiner’s relationship, referred to as the law of triads, seemed to work for a few elements, it was dismissed as coincidence.

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. Table 4.2 shows homolous series.

4. Newland's law of octaves

John A.R. Newland, an English chemist, found in 1865 that if the elements known at that time were arranged in the increasing order of their atomic weights, the properties of every eighth element were similar to those of the first one. Newland called it as law of octaves. Some of octaves formed by Newland are depicted in the table.

5. Lother Meyer's arrangement of elements

In 1869, Lother Meyer a German chemist plotted various physical properties of elements.

For example: Atomic volume, density, melting point, boiling point, thermal conductivity etc. were 'plotted 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.

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 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 a 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 lower atomic weight than that of tellurium was placed in group VII along with fluorine, chlorine, bromine because of the similarities in properties. It is indicated in Fig.

At the sametime, keeping the primary aim of arranging the elements of Mendeleev 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 silicon, and called these elements as Eka aluminium and Eka-silicon which were discovered afterwords. Mendeleev predicted not only the existence of gallium and germanium, but 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.