Presentation credits: Mr. Shivam Saha Dr. Tathagata Deb NCERT gives a brief classification of elements done by the following stalwarts: Dobereiner’s triads were groups of elements with similar properties that were identified by the German chemist Johann Wolfgang Dobereiner. He observed that groups of three elements (triads) could be formed in which all the elements shared similar physical and chemical properties. Dobereiner stated in his law of triads that the arithmetic mean of the atomic masses of the first and third element in a triad would be approximately equal to the atomic mass of the second element in that triad. The next reported attempt to classify elements was made by a French geologist, A.E.B. de Chancourtois in 1862. He arranged the then known elements in order of increasing atomic weights and made a cylindrical table of elements to display the periodic recurrence of properties. This also did not attract much attention. In the year 1864, the British chemist John Newlands attempted the 62 elements known at that time. He arranged them in an ascending order based on their atomic masses and observed that every 8th element had similar properties. On the basis of this observation, Newland’s law of octaves was formulated. The law of octaves states that every eighth element has similar properties when the elements are arranged in the increasing order of their atomic masses. Lothar Meyer's Curve: 1. Plotted the atomic volume of the elements against the atomic weight, and found that the chemical properties of the element recur periodically. (elements having similar properties will occupy the same position in the curve). 2. The elements occupying the corresponding position of the curve shows similar chemical properties. Dmitri Ivanovich Mendeléev, a Russian chemist, was the most important contributor to the early development of the periodic table. Many periodic tables were made but the most important one was the Mendeleev periodic table. In 1869, after the rejection of Newlands Octave Law, Mendeleev Periodic table came into the picture. In Mendeleev’s periodic table, elements were arranged on the basis of the fundamental property, atomic mass, and chemical properties. During Mendeleev’s work, only 63 elements were known. After studying the properties of every element, Mendeleev found that the properties of elements were related to atomic mass in a periodic way. He arranged the elements such that elements with similar properties fell into the same vertical columns of the periodic table.