Question

Which of the following can form Dobereiner’s triads 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

• Dobereiner 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.
• From the given options, the only element that can form a Dobereiner's Triad, along with lithium and sodium is only pottassium.
• If A, B and C are Dobereiner's Triads, $\text{Atomic mass of B}=\frac{\text{Atomic mass of A}+\text{Atomic mass of C}}{2}$