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classification of element
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Answer 1

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Definitions of groups, periods, alkali metals, alkaline earth metals, halogens, and noble gases. How metals, non-metals, and metalloids can be identified by the position on the periodic table.

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Answer 2

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Early Chemists - Arranged elements on basis of valency, metallic and non-metallic character.

• Method discard as elements showed variable valency and dual character.

Dobereiner [1815] - Arranged elements in increasing order of atomic weights.

• Elements were arranged in groups of three called triads.
• Atomic weight of the middle element was found generally to be the average of the other two.
• Method was discarded since it did not hold true for all elements.

Newland [1864] - Arranged elements in increasing order of atomic weights.

• Elements were arranged in a series of eight.
• Properties of every eight element was found to be a repetition of the first element.
• Method was discarded as it failed to leave spaces for undiscovered elements.

Mendeleeff [1869] - Arranged elements in increasing order of atomic weights.

• Elements were arranged in the form of a table called Mendeleef's Periodic Table.
• He stated that properties of elements were periodic functions of their atomic weights.
• Method could not justify position of certain elements, rare earths and isotopes.

Moseley [1912] - Arranged elements in increasing order of atomic numbers.

• Elements were arranged in a modified table called Modern Periodic Table.
• He stated that properties of elements were periodic function of their atomic numbers.
• Method removed most of the defects of Mendeleef's Periodic Table.