Dobereiner’s traids When elements are arranged in the. Order of increasing atomic masses, group of three elements known as traids, having similar properties are obtained. The middle element of the traid being equal to the arithmetic mean of the atomic masses of the other two elements. Following are the examples. 1. The alkali Metal group Elements Symbols Atomic masses Lithium Li 7 Sodium Na 23 Potassium K 39 2. Alkaline Earth metal group Elements Symbols Atomic masses Calcium Ca 40 Strontium Sr 88 Barium Ba 137 3. The halogen group Elements Elements Symbols Atomic masses Chlorine Cl 35.5 Bromine
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