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
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?
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.
The atomic masses of three elements X, Y, and Z having similar chemical properties are 7, 23, and 39 respectively.
(a) Calculate the average atomic mass of elements X and Z.
(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?
(d) What could the elements X, Y, and Z be?
(e) Give another example of a set of elements that can be classified according to this law. Ans.
(b) The average atomic mass of elements X and Z is equal to the atomic mass of element Y
(c) Dobereiner’s law of triads
(d) X is lithium, Y is sodium, Z is potassium
(e) Chlorine, Bromine, Iodine
In the following groups of elements be classified as Dobereiner’s triads?
(a) Na, Si, Cl (b) Be, Mg, Ca
Give the reason for your answer.
(Atomic masses : Be 9 ; Na 23 ; Mg 24 ; Si 28 ; Cl 35.5 ; Ca 40) .