Question

Identify the option that is not one of the Dobereiner's triad?
1..Chlorine-35, Bromine-80, lodine 127
2..Potassium-69, Chlorine-35.5, Bromine-80
3..Calcium-40, Strontium-88. Barium-137
4..Lithium-6, Sodium-22, Potassium-39

please answer fast​

Answer 1

$\huge\mathcal\colorbox{lavender}{answer}$

Option 2 doesn't belong to his triads

Potassium-69, Chlorine-35.5, Bromine-80 - the middle element's atomic weight isn't nearly equal to the average of the other two