Which of the following is (are) Dobereiner’s triads?
A. P, As, Sb
B. Cu, Ag, Au
C. Fe, Co, Ni
D. S, Se, Te

Hint: According to Dobereiner’s triads, atomic weight of the middle element is equal to the average of the preceding and the next element. So we will check the atomic weight of all the elements and check which suits the above criteria. Triad means a set of 3.

Complete step-by-step answer:In 1817 Dobereiner’s triad rule was arrived, according to this rule atomic weight of the middle element is nearly equal to the average weight of the first and the third element. Let us check which of the following group of elements follows this traid or Trend:
P, As, Sb
These three elements are Phosphorus, arsenic and antimony.
The atomic weight of phosphorus is 31.
The atomic weight of arsenic is \[74.9\]
The atomic weight of antimony is 122.
The atomic mass of middle element using Dobereiner’s triad rule should be: \[\dfrac{{31 + 122}}{2} = 76.9\]
which is not close to the actual mass and hence it does not follow Dobereiner’s triad rule.
Cu, Ag, Au
These three elements are copper, silver and gold.
The atomic weight of copper is \[63.5\]
The atomic weight of silver is \[108\]
The atomic weight of gold is 197
The atomic mass of middle element using Dobereiner’s triad rule should be: \[\dfrac{{63 + 197}}{2} = 130\]
which is not similar to the actual mass and hence it does not follow Dobereiner’s triad rule.
Fe, Co, Ni
These three elements are iron, cobalt and nickel.
The atomic weight of iron is \[56\]
The atomic weight of cobalt is \[58.9\]
The atomic weight of nickel is \[58.6\]
The atomic mass of middle element using Dobereiner’s triad rule should be: \[\dfrac{{56 + 58.9}}{2} = 57.3\]
which is quite similar to the actual mass and hence it follows Dobereiner’s triad rule.
S, Se, Te
These three elements are sulphur, selenium and tellurium.
The atomic weight of iron is 32
The atomic weight of cobalt is \[79.2\]
The atomic weight of nickel is \[129.2\]
The atomic mass of middle element using Dobereiner’s triad rule should be: \[\dfrac{{32 + 129.2}}{2} = 80.6\]
which is quite similar to the actual mass and hence it follows Dobereiner’s triad rule.

Hence, the correct options are C and D.

Note: bVarious classifications were given earlier to arrange the known elements according to the property in such a way that elements of similar properties are grouped together in a tabular form. First of all elements were classified or distributed as metals and non-metals. Then there was a hypothesis that elements are made up of hydrogen.