Answer :

1. The German Chemist, J.W Dobereiner was the first to consider the idea of trends among properties of elements.

i. He made group of three elements each which are having similar properties and called them triads.

ii. He showed that the atomic mass of the middle element was approx. equal to the mean of atomic masses of other two elements.

iii. For example:

2. John Alexander Newlands proposed the law of octaves.

Law of octaves:

i. Law of octaves states that “every eighth element has properties similar to the first element”

ii. In modern periodic table, any element of the same period has similar properties.

iii. For Example-Mg and Ca exhibit similar properties as they both belong to the same group.

3. The German scientist Julius Lothar Meyer plotted a graph of atomic volume against the atomic mass of the elements.

He noticed that:

i. Elements of same properties remained at same positions.

ii. He concluded that atomic mass is the fundamental property of elements.

4. Chan Courtoise put forward the idea of “Telluric helix” to classify elements.

i. He arranged the atoms in spirally ascending order of their atomic masses in a cylindrical way.

ii. He noticed that elements of similar properties will line up vertically.

5. John Dalton discovered an Atomic Theory. He concluded that each element has a fixed mass.

6. Antoine Lavoiser classify the known elements into metals and non-metals. But by that time, he was not able to classify metalloids which show the properties of both metals and non-metals.

7. Dmitri Mendeleev invented modern periodic law. Modern periodic law states that “the chemical and physical properties of elements are periodic function of their atomic numbers”.

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