Periodic Classification of Elements

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The earliest classification of elements was into metals and non-metals. This helped in grouping elements with similar general properties.

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Nearly 60 elements were known by then, and their compounds were also very large in number. It became difficult to study each element separately, so classification helped in systematic study.

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William Prout's work in 1815 showed that the atomic mass of an element remains constant. This made atomic mass a useful basis for classification.

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When elements are arranged in order of increasing atomic mass, the atomic mass of the middle element is nearly equal to the arithmetic mean of the other two, and its properties are intermediate betwee…

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Atomic masses: lithium = 7 u, potassium = 39 u. Mean = (7 + 39) / 2 = 23 u. Sodium has atomic mass 23 u, so the triad fits the rule.

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Atomic masses: calcium = 40 u, barium = 137 u. Mean = (40 + 137) / 2 = 88.5 u. Strontium has atomic mass about 88 u, so the triad fits the rule.

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Atomic masses: chlorine = 35.5 u, iodine = 127 u. Mean = (35.5 + 127) / 2 = 81.25 u. Bromine has atomic mass about 80 u, so the triad fits the rule.

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Only a few elements could be arranged into triads. So, the system was useful but limited.