Dobereiner could identify only three triads from the elements known at that time. So, all the elements could not be arranged in triads. For Example: The three elements, nitrogen, phosphorus and arsenic have similar properties. Therefore they can be regarded to form a triad. However, the actual atomic mass if the middle element, phosphorus(P)(31.0u) is much lower than the average or mean(44.45u)of the atomic masses of nitrogen and arsenic. Thus, these three elements do not constitute a Dobereiner triad in spite of their similar chemical properties.
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