Before the beginning of the eighteenth century when there were 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 become difficult to study them. So scientist felt the need of simple method to facilitate the study of the properties of various elements and their compounds. After numerous attempts they got success and elements were arranged in such manner that similar elements were grouped together different elements were separated. This arrangement of elements is known as classification of elements which led to the formation of periodic table. 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 is 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.