PERIODIC CLASSIFICATION OF ELEMENTS – Introduction, Dobereiner’s Triads
Till 18th centuries, only 30 elements were known. By the year 1865, 63 elements were discovered. Till date , 118 elements have been discovered and many more may be known in future.
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Need of classification –
It is practically difficult to learn properties and uses of each element separately. So, attempts
have made for systematic study of such large elements with ease. The basis of Classification is
similarities in properties.
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Earlier attempts –
♦ Table of relative atomic weights given by Dalton
J.M. Dobereiner was first to classify elements into some groups having sets of three elements each, called triads. He recognized the relation between atomic mass and their properties.
When elements were arranged in the order of increasing atomic masses and similar properties, the relative atomic mass of the middle element in each triad was close to the average of the relative atomic masses of the other two elements.
Triad I Triad II Triad III
Element Li Na K Cl Br I Ca Sr Ba
Atomic mass – 6.9 23 39 35.5 79.9 126.9 40.1 87.6 137.3
Average at. mass
of 1st and 3rd 6.9 + 39 / 2 35.5 + 126.9 / 2 40.1 + 137.3 / 2
elements of a triad
♦ Dobereiner made the first observation on platinum as catalyst.
Demerits of the Dobereiner’s Triads
- A large number of elements having similar properties could not be grouped into triads.
Example – N, P and As have similar properties but the average of atomic masses of N and As is not
equal to the atomic mass of P.
Elements – N P As
Atomic mass – 14 31 74.9
Average atomic mass of N and As = 14 + 74.9 /2 = 44.45 (not equal to the at. mass of P i.e. 31)
- It was possible that quite dissimilar elements could be grouped into a triad.
- Dobereiner’s Triads had limited use as only three triads could be arranged.
Although Dobereiner’s Triads was not found to be very useful, even the only advantage of Dobereiner’s research was that it encouraged other chemists to correlate the properties of elements with their atomic masses.