(a)Yes,Dobereiners triads also exist in the columns of Newlands’ Octaves. Consider the elements lithium (Li), sodium (Na) and potassiu m (K) which are present in the second column of Newlands’ classification of elements. Now, if we start with lithium as the 1 st element, then the 8 th element from it is sodium, and according to Newlands’ law of octaves, the properties of 8 th element , sodium should be similar to thos e of the 1 st element, lithium. Again, if we take sodium as the 1 st el ement , then the 8 th element from it is potassium, and according to Newlands ‘ law of octaves, the properties of 8 th element, potassium should be similar to those of the 1 st element, sodium. This means that according to Newlands’ law of octaves, the elements lithium, sodium and potassium should have similar chemical properties. We also know that lithium, sodium and potassium form a Dobereiner’s triad having similar chemical properties. From this, we conclude that Dobereiners triads also exist in the columns of Newlands Octaves.
(b) The main limitation of Dobereiner’s classification of elements was that it failed to arrange all the then known elements in the form of triads of elements having similar chemical properties. Dobereiner could identify only three triads from the elements known at that time. So, his classification of elements was not much successful. Another limitation was that Dobereiner failed to explain the relation between atomic masses of elements and their chemical properties.
(c) Newlands’ law of octaves for the classification of elements had the following limitations:
( i ) Newlands’ law of octaves was applicable to the classification of elements up to calcium only. After calcium, every eighth element did not possess the properties similar to that of the first element. Thus, this law worked well with lighter elements only.
(ii) Newlands assumed that only 56 elements existed in nature and no more elements would be discovered in the future. But later on, several new elements were discovered whose properties did not fit into Newlands’ law of octaves.
(iii) In order to fit elements into his table, Newlands put even two elements together in one slot and that too in the column of unlike elements having very different properties. For example, the two elements cobalt (Co) and nickel (Ni) were put together in just one slot and that too in the column of elements like fluorine, chlorine and bromine which have very different properties from these elements.