Before the beginning of the eighteenth century when there are only 30 elements known, it was easier to study and remember their properties.
 In later years when number of elements discovered were increased then it became difficult to study them. So scientist fell the need of simple
 method to facilitale the study of the properties of various elements and their compounds. After numerous attempts they got success & elements
 were arranged in such manner that similar elements were grouped together and different elements were separated. This arrangement of elements
 is known as classification of elements which led to the formation of periodic table.
   Periodic table may be defined as "the arrangement of all the known elements according to their properties in such way that the elements
   of similar properties are grouped together in a tabular form."
   Earlier attempts of classification of elements (development of periodic table) :
   Earlier attempts to classify the elements resulted in grouping as metals and non-metals. Later on they were classified on the basis of their atomic masses.

Dobereiner Triads rule

    In 1817, Johann walfgang dobereiner. A German chemist, arranged the elements as group of three elements and in a manner that the atomic mass of
   middle element was roughly the average of the atomic masses of the other two elements of the triad.
   Example : Element :        Lithium,    Sodium       and     Potassium.
                  Atomic mass        7             23             39
    Average of the atomic masses of Lithium and Potassium is only three triads could be arranged in this manner at that time. They were :

Short coming of Dobereiner's triads rule

    This classification was not found satisfactory as it could be applied to the limited number of elements.
    Now a days some more triads have been made they are
    (i)    Potassium       Rubidium        Cesium
            K                   Rb                  Cs
    (ii)    Phosphorus    Arsenic           Antimony
            P                   As                  Sb
    (iii)    Sulphur        Se
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