What are Dobereiner’s Triads? Give examples.

Table of Content

• Need of Classification
• What are Dobereiner’s Triads?
• Examples of Dobereiner’s Triads
• Limitations/ Demerits of Dobereiner’s Triads
• Frequently Asked Questions – FAQs
• Quiz
• Next – Newland’s Law of Octaves and Limitations

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.

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.

Earlier attempts –

♦  Grouping as metals and non-metals.

♦  Table of relative atomic weights given by Dalton

Dobereiner’s Triads

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

1. 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)

2. It was possible that quite dissimilar elements could be grouped into a triad.
3. 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.

Frequently Asked Questions – FAQs

1. Did Dobereiner’s law of traid also exist in the columns of Newland’s octaves? Compare and find out. (NCERT)
2. Why was Dobereiner’s triad lost its relevance? Give three reasons in support of your
answer.
3. Define triad.

Check your understanding –

9

Created on July 18, 2022 By admin

Dobereiner’s Triads -Periodic classification of Elements

1 / 5

What is the basis of classification of elements?

(a) Atomic number

(b) Atomic size

(c) Atomic mass

(d) Similarities in properties

2 / 5

Why is the classification of elements needed?

(a) To study properties of elements separately

(b) Systematic study of large number of elements with ease

(c) To arrange the elements in increasing order of atomic number

(d) To arrange the elements in increasing order of atomic mass

3 / 5

Traid is –

(a) set of tiny three elements

(b) set of tiny three elements having similar properties

(c) set of tiny three elements having dissimilar properties

(d) set of tiny four elements

4 / 5

The set of N, P and As could not be explained by Dobereiner’s traid –

(a) because they have dissimilar properties.

(b) because they have similar properties.

(c) average mass of N and As is not equal to the atomic mass of P.

(d) because they are not arranged in increasing order of atomic masses.

5 / 5

Which of the following traid is not Dobereiner’s traid?

(a) Li Na K

(b) Cl Br I

(c) Ca Sr Ba

(d) Li Be B

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NEWLANDS’ LAW OF OCTAVES