Third Triad contains which elements? I. Ir II. Os III. Pd IV. Pt

• A. I, II, III
• B. I, II, IV
• C. I, III, IV
• D. II, III, IV

Answer: (B)

Dobereiner’s triads group three elements that share similar properties, with the middle element’s atomic number equal to the average of the outer two. In this case, the heavy platinum-group trio consists of Osmium (atomic number 76), Iridium (77), and Platinum (78). Since 77 is the average of 76 and 78, Iridium sits between Osmium and Platinum, forming the triad. Palladium doesn’t belong to this trio because its atomic number doesn’t fit the average relationship with these two; it’s not between Osmium and Platinum in this pattern. So the elements in the third triad are Osmium, Iridium, and Platinum.