Question

If the two members of a Dobereiner triad are
phosphorus and antimony, the third member of
this triad is –
(a) arsenic (b) sulphur
(c) iodine (d) calcium

Answer 1

Answer:

according to me it would be option (a)

Answer 2

(a) arsenic

Explanation:

According to Dobereiner, the atomic mass of the middle element is nearly equal to the arithmetic mean of the other two.

Atomic mass of the third element is =   (31 + 121.75) / 2  = 76.37.

Atomic mass of arsenic is 75 u.  This is closest to the value required. So Arsenic is the third element.

Option A is the answer.