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Early Attempts at the Classification of Elements - Dobereiner’s Triads

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DOBEREINERS TRIADS:-

In the year 1817, Johann Wolfgang Döbereiner, a German chemist, tried to arrange the elements with similar properties into groups. He identified some groups having three elements each. So he called these groups ‘triads’.

When the three elements in a triad were written in the order of increasing atomic masses; the atomic mass of the middle element was roughly the average of the atomic masses of the other two elements.

For example:-

lithium (Li)

6.9

sodium (Na)

23.0

potassium (K)

39.0

The average of the atomic masses of the first and the last element is 22.95, which is almost equal to the middle element.

Limitation:-

Döbereiner could identify only three triads from the elements known at that time. Hence, this system of classification into triads was not found to be useful.

These were the only triads recognised from the elements known at that time.

(i) Li, Na, K

(ii) Ca, Sr, Ba

(iii) Cl, Br, I

 

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