Periodic Classification of Elements:--
NEED FOR PERIODIC CLASSIFICATION:--
Elements were classified so that they could be studied easily and in an organised manner. The earliest attempt to classify the elements resulted in grouping the then known elements as metals and non-metals. Later further classifications were tried out as our knowledge of elements and their properties increased.
DOBEREINER's TRAIDS :-
In the year 1817, Dobereiner, a German chemist, tried to arrange the elements with similar properties into groups. When elements were arranged in the order of increasing atomic masses, groups of three elements (known as triads), having similar chemical properties were obtained. The atomic masses of the middle element of the triad was roughly the average of the atomic masses of the other two elements.
NEWLAND'S LAW OF OCTAVES :-
Newland arranged the then known elements in the order of increasing atomic masses and found that the properties of every 8th element is similar to that of the 1st element. He compared this to the octaves found in music and called it the 'Law of Octaves'. For example, the properties of lithium (Li) and sodium (Na) were found to be the same.