PERIODIC TABLE AND PERIODICITY OF PROPERTIES
Q.1 What are the contributions of Dobereiner, Newland, Luthar Meyer and Mendeleef in the present shape of long form periodic table ? Dobereiner’s Triads :
In 1829, a German scientist, John Dobereiner classified specific elements into groups in order of increasing atomic masses. Each group has three elements. He named these groups as triads.He noticed that the atomic mass of the middle elements was found to be approximately equal to the arithmetic mean of the other two elements.
The Dobereiner’s method of classification was failed in arranging all elements. The elements were selected out of those known at that time. Therefore the idea of triads was rejected. Newland’s Law of Octaves :
In 1864, John Newland, an English chemist arranged the elements in the order of their increasing atomic masses. He noticed that the properties of the elements were repeated at every eighth element.
Newland called this as the law of octaves. Newland’s classification of a few elements is shown in the following table. Note:The numbers in parentheses are atomic masses.
Table: Newland’s law of octaves.
From the above table , we can see that the properties of sodium (11Na) and potassium (19K) are similar to those of lithium (3Li). In the same way, chlorine (17Cl) resembles fluorine (9F).
Newland’s classification failed badly while dealing with the heavier elements beyond calcium .
Luthar Meyer’s Atomic Volum Curves :
In 1869, a German chemist, Julius Luthar Meyer drew a graph of atomic volume versus atomic mass for a variety of elements. He found that the elements with similar properties occupied alike positions on the curve as shown in following Fig.
For instance, alkali metals (Li, Na, K, Rb, Cs, etc.) occupy the peak positions. Hence, Luthar Meyer found a periodicity in the properties of the elements with atomic mass.
Luthar Meyer’s classification for the first time confirmed a definite pattern and periodicity in the properties of the elements.
THE MENDELEEF’S PERIODIC LAW
the Russian chemist D’ mitri Mendeleef studied the chemical properties of all the elements know at that time and their compounds. Mendeleef classified elements in the order of increasing atomic masses. He found that the elements with similar properties came into sight at regular intervals. On the basis of these observations , In 1869, Mendeleef stated his famous periodic law as follows : The properties of the elements are a periodi function of their atomic masses. OR
when the elements were arranged in the increasing order of their atomic masses. The elements with similar properties are repeated at regular intervals.
Q.2 Write a short note on the classification of elements in Mendeleef’s Periodic Table. Mendeleef’s Periodic Table :
In Mendeleef’s periodic table, the elements with similar properties were positioned in vertical columns and the horizontal rows called groups and periods respectively.
In the classification, Mendeleef laid more emphasize on the similarity in the properties of the elements. The order of increasing atomic masses was not strictly followed. Mendeleef’s periodic table was an excellent attempt for the classification of elements, but it also had certain defects. Q.3 What is the contribution of Moseley in Modern periodic Law ?
H.G Moseley, in 1913, observed that when an element was bombarded with electrons in vacuum, X-rays were produced. The wavelength of X-rays was associated with the atomic number of the element. Hence, Moseley concluded that it is the atomic number and not the atomic mass which should find out the position of an element in the periodic table. On the basis of these discoveries Moseley stated the Modern form of the periodic law as follows :
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