Edusaint

• In the year 1829, a German chemist Dobereiner observed that certain elements had similar properties and that he could put them together in groups of three elements each.
• When elements are arranged in the order of increasing atomic masses, groups of three elements (known as triads), having similar chemical properties are obtained.
• The atomic mass of the middle element of the triad being equal to the arithmetic mean of the atomic masses of the other two elements.
• The Alkali Metal Group. The elements lithium, sodium and potassium have similar chemical properties and form a triad.
• For example :
• All these elements are metals,
•  All of them react with water to form alkalis and hydrogen gas, and
• All of them have a valency of 1 (they are monovalent).
• So, due to their similar chemical properties, the elements lithium, sodium, and potassium were put in one group called alkali metal group or alkali metal family and form a Dobereiner’s triad
• Dobereneir’s idea of classification of elements into triads did not receive wide acceptance as he could arrange only 9 elements in triad form ### Now Think of the Questions

1. A, B, and C are the elements of a Dobereiner’s triad. If the atomic mass of A is 7 and that of C is 39, what should be the atomic mass of B?
Ans. 23
2. State one example of a Dobereiner’s triad, showing in it that the atomic mass of middle element is half-way between those of the other two.
Ans.
3. The atomic masses of three elements X, Y, and Z having similar chemical properties are 7, 23, and 39 respectively.
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a) Calculate the average atomic mass of elements X and Z.
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b) How does the average atomic mass of elements X and Z compare with the atomic mass of element Y?
(
c) Which law of classification of elements is illustrated by this example?
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d) What could the elements X, Y, and Z be?
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e) Give another example of a set of elements that can be classified according to this law.

Ans.
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a) 23
(
b) The average atomic mass of elements X and Z is equal to the atomic mass of element Y
(