In a Dobereiner's triad consisting of three elements A, B, and C, the sum of atomic masses of A and B is 115 and the average of atomic masses A and C is 81. Find the atomic mass of A, B, and C. Predict the names of these elements.

Explanation

Given: `A+B = 115 ........(1)`
  and `(A+C)//2 = 81 .............(2)`
  Since A, B, and C form a Dobereiner.s triad, average of atomic masses of A and C must be equal to atomic mass of B, i.e., B = 81. Using this in (1), we get,
`A + 81 = 115 or A = 115-81 = 34.`
  Substituting the value of A in (2), we get,
` 34+C = 2 xx 81 = 162 or C = 162 - 34 = 128.`
  Thus, atomic masses are: A = 34, B = 81, and C = 128.
Since we know that chlorine has a atomic mass of about 34, we can guess that this triad must be chlorine, bromine, and iodine.