Question

Which of the following can form a Döbereiner's triad with lithium (atomic mass = 6.9 u) and sodium (atomic mass = 23 u)?

• A. Sulphur, atomic mass = 32 u
• B. Chlorine, atomic mass = 35 u
• C. Calcium, atomic mass = 40 u
• D. Potassium, atomic mass = 39 u

Answer 1

The correct option is D

Potassium, atomic mass = 39 u

• Döbereiner arranged elements in groups of three, called triads, in the increasing order of their atomic masses. In a triad, the atomic mass of the middle element was roughly equal to the average of the atomic masses of the other two elements.
• If lithium and sodium form a Döbereiner's triad with an element X, $\text{Atomic mass of sodium}=\frac{\text{Atomic mass of Lithium}+\text{Atomic mass of element X}}{2}$

23 $=\frac{\text{6.9}+\text{Atomic mass of X}}{2}$
Atomic mass of X = 39
Hence, potassium forms a Döbereiner's triad with lithium and sodium.