The first classification of elements was as metals and non-metals. This served only limited purpose mainly because of two reasons: (i). All the elements were grouped in to these two classes only. The groupcontaining metals was very big. (ii) Some elements showed properties of both-metals and non-metals and they could not be placed in any of the two classes.
2. All earlier attempts on the classification of the elements were based on their atomic weights.
3. The first attempt towards the classification of elements was made by Johann Dobereiner, a German chemist in 1829. He made sets of three elements which showed similar chemical properties and he called it triads.
4. The Triad was approximately the mean of the atomic weights of the other two members and the properties of the middle element were in between those of the other two members.
5. The average of the atomic masses of Li and K =1/2(7+39 )= 23 (Na)
5. The major drawback of Dobereiner classification was that the concept of triads could be applied to limited number of elements. It was also possible to group quite dissimilar elements into triads.
6. Lithium, Sodium, Potassium are called Alkalies metal as they react with metal and form alkalies (Caustic solution)
7. Calcium, Strontium, Bromine are called Alkalies Earth metal as they react with metal and form salt .
as their oxide are alkalies in nature and exist in the Earth
8. Chlorine, Bromine, Iodine are called Halogen as their oxide are alkalies in nature and existin the Earth
9. In 1864 John Alexander Newland, an English chemist noticed that “when elements are arranged in the increasing order of their atomic masses. He found that "every eighth element had properties similar to the first element.”
10. Newland called this as the Law of Octaves due to its similarity with musical notes .For Solved CBSE Exam questions Visit JSUNILTUTORIAL
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10th Periodic Classification of Elements (13 posts by Chemistry adda)