If calcium and strontium are the first two members of the Dobereiner triads, which one is the third element?

(atomic masses are mentioned in the bracket below)

 [ Ca= 40 u, Sr= 87.6 u, Ba=137.23 u, Cs=132.9 u, Mg=24.3 u, Na=22.9 u ]

 The atomic mass of calcium and strontium is 40 and  87.62

 Let us consider x as the third element.

 According to Dobereiner's triads rule-

$\frac{40+x}{2}=87.62$

$40+x=87.62\times 2$

$x=175.24-40$

$x=135.24 u$

The value of x is approximately equal to the atomic mass of Barium.

Hence, we can say that the third element of this triad is 'Barium'.