Periodic Table

HISTORICAL BACKGROUND OF CLASSIFICATION OF ELEMENTS

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The historical outcome of the concerted efforts of the scientists, J. W. Dobereiner, B. Chancourtois, J. A. R. Newlands and D. I. Mendeleev, is the modern periodic table-an essential tool to study the study the physical and chemical properties of elements.

• Dobereiner’s law of triads: In 1817, the German scientist Dobereiner first tried to simplify the study of chemistry of elements by arranging them on the basis of atomic weights. He stated that in a group of three chemically similar elements, called triads, the atomic weight of the middle element was approximately the arithmetic mean of those of the other two elements. This was called Dobereiner’s law of triads. Some familiar triads, based on this law, are:

  

  Law of triads

   From the tables it is observed that the atomic weight of sodium (Na) is the average of the atomic weights of lithium (Li) and potassium (K) [7+39/2=23]. This numerical relationship exists also in the case of other triads. This law was applicable only in case of a limited number of elements and hence failed to be of much help in the desired classification of elements.
  Using the atomic weights, determined by Cannizzaro’s method’ Chancourtois in 1862 arranged the elements in the order of their increasing atomic weights on a spiral line or helix, drawn on the surface of a vertical cylinder, inclined at 45° and noticed that in the tellurique screw, chemically similar elements were aligned in the same vertical line, displaying the recurrence of identical chemical properties.

• Newlands’ law of octaves: Upon arranging the then known elements in the ascending order of their atomic weights, Newlands, the British scientist observed in 1864 that, the 8th element, starting from a given one, is a kind of repetition of the 1st, like the 8th note in an octave of music and he called this regularity the law of octaves. 

  

  Law of octaves

   Starting from Li, the 8th element is Na and the 8th element following Na is K. In fact, there is a striking resemblance in properties among these elements which lie in the same vertical column. Similarly, F shows similarity with the 8th element following it, that is, Cl, in properties. The law of octaves was however found to be satisfactory in the case of light elements from hydrogen to calcium but in the case of heavy elements beyond calcium, it totally lost its validity and hence the law was discarded.

Mendeleev’s periodic law

In 1869, the celebrated Russian scientist, Mendeleev put forward his famous periodic law-a memorable event in the history of the systematic study of chemistry. The law states that the physical and chemical properties of the elements are periodic functions of their atomic weights. That is, if the elements are arranged in the order of increasing atomic weights, the properties of the elements change in a regular manner from one member to another but their properties are repeated after a definite interval. This phenomenon of periodic change in properties of elements is called periodicity of elements. The classification of elements based on Mendeleev’s periodic law is called periodic classification and the arrangement of elements, he made, in the form of a table showing periodic variation of their properties with atomic weight is known as Mendeleev’s periodic table.

Periodic table

 Mendeleev’s periodic table

At the time of the publication of Mendeleev’s periodic table, inert gases had not been discovered. Till then only 63 elements had been discovered. So, in the original table, many places were left vacant. Elements subsequently discovered have been placed in the positions, suggested by Mendeleev.

• Period: Mendeleev, while arranging the elements on the basis of steadily increasing atomic weights, placed some elements in horizontal rows. These horizontal rows are called periods. Modern version of Mendeleev’s periodic table consists of 7 such periods.
• Even and odd series: Each of the 4th, 5th and 6th periods is further divided into even and odd series. Taking the hydrogen-series as the 1st series, the elements occupying places in the series of even serial numbers are said to be the elements of even series and the elements placed in the series of odd serial numbers are called elements of the odd series.
• group: While placing the elements successively in horizontal rows on the basis of increasing atomic weight, Mendeleev observed that the elements having similar chemical properties, quite surprisingly, had their positions in the same vertical column. These vertical columns are known as groups. The modern version of Mendeleev’s periodic table contains 9 groups. These groups have been designated by 0 (zero) and the Roman numerals I to VIII. Inert elements have been included in the additional group (group 0), appended to the main table.
• sub-group: Excepting group VIII and group 0, each of the other groups has been divided into sub-groups A and B. In the case of long periods, elements of the even series have been placed in sub-group A and those of the odd series in sub-group B. In the case of short periods, elements belonging to the 1st and 2nd groups have been placed in sub-group A nad the elements of other groups in sub-group B.