Up to which element was the Law of Octaves found to be applicable?

Define Newlands' law of Octaves.

Write true or false for the statement: In order to fit elements into his table,Newlands adjusted three elements in the same slot,but also put some unlike elements in the same slot.

Write true or false for the statement: It was assumed by Newlands that only $63$ elements existed in nature and no more elements would be discovered in the future.

Elements have been arranged in the following sequence on the basis of their increasing atomic masses.
$F, Na, Mg, Al, Si, P, S, Cl, Ar, K$
$(a)$ Pick two sets of elements which have similar properties.
$(b)$ The given sequence represents which law of classification of elements?

$D$öbereiner's system of classification into triads was not found to be useful as he could identify only: