<P> Lavoisier's Traité Élémentaire de Chimie (Elementary Treatise of Chemistry), which was written in 1789 and first translated into English by the writer Robert Kerr, is considered to be the first modern textbook about chemistry . Lavoisier defined an element as a substance that cannot be broken down into a simpler substance by a chemical reaction . This simple definition served for a century and lasted until the discovery of subatomic particles . Lavoisier's book contained a list of "simple substances" that Lavoisier believed could not be broken down further, which included oxygen, nitrogen, hydrogen, phosphorus, mercury, zinc and sulfur, which formed the basis for the modern list of elements . Lavoisier's list also included' light' and' caloric', which at the time were believed to be material substances . He has classified these substances into metals and non metals . While many leading chemists refused to believe Lavoisier's new revelations, the Elementary Treatise was written well enough to convince the younger generation . However, Lavoisier's descriptions of his elements lack completeness, as he only classified them as metals and non-metals . </P> <P> In 1815, the English physician and chemist William Prout noticed that atomic weights seemed to be multiples of that of hydrogen . </P> <P> In 1817, Johann Wolfgang Döbereiner, a chemist, began to formulate one of the earliest attempts to classify the elements . In 1829, he found that he could form some of the elements into groups of three, with the members of each group having related properties . He termed these groups triads . </P> <P> Definition of Triad law: - "Chemically analogous elements arranged in increasing order of their atomic weights formed well marked groups of three called Triads in which the atomic weight of the middle element was found to be generally the arithmetic mean of the atomic weight of the other two elements in the triad . </P>

Who was the first chemist that tried to group elements based on their properties