(i) (A) because Dobereiner stated in his law of triads that the arithmetic mean of the atomic masses of the first and third element in a triad would be approximately equal to the atomic mass of the second element in that triad.
(ii) Mendeleev placed the elements in order of increasing atomic masses of the elements and their physical and chemical properties. Among chemical properties, Mendeleev concentrated on the compounds formed by elements with oxygen and hydrogen. He selected hydrogen and oxygen as they are very reactive and formed compounds with most elements. The formulae of the hydrides and oxides formed by an element were treated as one of the basic properties of an element for its classification, hence, a, b, c are placed in different category.
(c) No, Newland law of octaves is not applicable as it states that the chemical elements are arranged according to increasing atomic masses, those with similar physical and chemical properties occur after each interval of eighth element.
Newland law of octave.
It was found that the Law of Octaves was applicable only up to calcium, as after calcium every eighth element did not possess properties similar to that of the first.
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