chapter-5 ( class 10) Periodic Classification of Elements INTRO *In the universe 115 elements have been discovered till today. *Each of these elements possesses different properties. *it is difficult to understand and use the properties of each elements at a time. *hence attempts were made to discover ways to learn the properties of elements in a systematic order. Doberenier's Triads :- in 1829, more than 30 elements were known . Doberenier German scientist made some group of three elements each and called them TRIADS. Characteristics:- atomic mass of second elements of a triad in nearly equal to the arithmetic mean of atomic masses of other two elements. *Elements in triads have similar properties. It is found that atomic mass of Sodium is arithmetic mean of first elements lithium and third element potassium and the properties of sodium were ...