There are 115 elements known at present and it is very difficult to study the properties of all these elements separately. So all the elements have been divided into a few groups in such a way that elements in the same group have similar properties. This is the reason for the periodic classification of elements.
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. These groups of three elements were called triads. All the three elements of a Particular triad had similar chemical properties.
According to Dobereiner law of triads. 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.
|Elements of the triad||Symbols||Atomic masses|
- The Alkali Metal Group: The elements lithium, sodium and potassium have similar chemical properties and form a triad. All these elements are metals. All of them react with water to form alkalis and hydrogen gas. All of them have a valency of 1 (they are monovalent)
- The Alkaline Earth Metal Group: The elements calcium, strontium and barium have similar chemical properties and form a triad. All these elements are metals. The oxides of all of them are alkaline in nature, and All these elements have a valency of 2 (they are divalent)
- The Halogen Group: The elements chlorine, bromine and iodine have similar chemical properties and form a triad. All these elements are non-metals. All these elements react with water to form acids, and All these elements have a valency of 1 (they are mono -valent).
The arithmetic mean of the atomic masses of the first and third Members of this triad (chlorine and iodine), it will come to be 35.5 + 127/2 = 81.2 and the actual atomic mass of the middle element of this triad (bromine) is 80. Thus the atomic mass of the middle element of this triad (bromine) is approximately equal to the arithmetic mean of the atomic masses of the first and third elements of the triad.
The limitation of Dobereiner’s classification was that it failed to Arrange all the then known elements in the form of triads of elements having similar chemical properties. Dobereiner could identify only three triads from the elements known at that time. So his classification of elements was not much successful.