If the first and third members of Dobereiner's triad are Chlorine and Iodine. What is the middle member of the triad?

(atomic mass of Cl=35.5 u, atomic mass of I=127 u)

The first and third members of the Dobereiner triads are Chlorine and Iodine and the middle members of the triads are Bromine.

The atomic mass of Chlorine and Iodine is 35.5 u and 126 u.

$\mathrm{Mean}=\frac{35.5+127}{2}$

$\mathrm{Mean}=81.25$

The mean value of the Chlorine and Iodine is equal to 81.25 u.

The mean value of the Chlorine and Iodine is approximately equal to the atomic number of the Bromine.

Hence, the third member of the triad is 'Bromine'.