Answer 1

According to Dobereiner's triad, the atomic mass of the second element is nearly equal to the average of the atomic masses of the first and the third elements provided the three elements are arranged in the increasing order of their atomic masses.

Na, Si, Cl:
Atomic mass of Na - 23, Si - 28, Cl - 35.
Average of atomic masses of Na and Cl = `(23 + 35)/(2) = 29`  which is nearly equal to the atomic mass of Si i.e. 28.
Thus, it can be considered as Dobereiner's triad.