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Dobereneir arranged elements in groups that wher called traids by him and he found that arithemetic mean of at.masses of the other two elements is equal to the at. mass of the middle element.
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Doberneir was great scientitst he arranged the elements of same properties  as 3 elements  as1 he took the average  mass of 1st and 3rd element which  gives the mass of 2nd element this is called dobrereneirs law of riads ghese elements are called dobrernier triads
 but this law is valid till 1 to 20 elements