CBSE ADDA: X Chemistry: Periodic Classification of elements 1. Periodic table: The table which classifies all the known elements on the basis of their properties in such a way that elements with similar properties are grouped together. The first classification of elements was as metals and non-metals. This served only limited purpose mainly because of two reasons: (i). All the elements were grouped in to these two classes only. The groupcontaining metals was very big. (ii) Some elements showed properties of both-metals and non-metals and they could not be placed in any of the two classes. 2. All earlier attempts on the classification of the elements were based on their atomic weights. 3. The first attempt towards the classification of elements was made by Johann Dobereiner, a German chemist in 1829. He made sets of three elements which showed similar chemical properties and he called it triads. 4. The Triad was approximately the mean of the atomic weights of the other two members and t...