- 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.
- 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.
(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) .