Question

Which of the following can form Dobereiner’s Triads: with Li(m = 6.9) and Na(m = 23)?

• A. K (m = 32)
• B. K (m = 37)
• C. K (m = 45)
• D. K (m = 39)

Answer 1

The correct option is D

K (m = 39)

By looking at the atomic masses of given elements and the options, you can tell that Lithium and Sodium are the first two elements of the triad. Thus 23 is the average mass. If you try with the given options, you will find that only K(m = 39) will satisfy the rule that average of the atomic masses of the first and the last element is the atomic mass of the middle element.