Question

Which of the following triads do not follow Dobereiner's law of triads?

• A. Li, Na, K
• B. Ca, Sr, Ba
• C. Be, Mg, Ca
• D. Cu, Ag, Au

Answer 1

The correct option is D

Cu, Ag, Au

• Dobereiner's Law of Triads - When elements are arranged in the order of increasing atomic masses, groups of three elements (known as triads), having similar chemical properties are obtained. The atomic mass of the middle element of the triad is equal to the arithmetic mean of the atomic masses of the other two elements.
• Dobereiner could identify only three triads from the elements known at that time.

Atomic weight of Li	Atomic weight of Na	Atomic weight of K
7	$\frac{7+39}{2}=23$	39
Atomic weight of Ca	Atomic weight of Sr	Atomic weight of Ba
40	$\frac{40+137}{2}=88$	137
Atomic weight of Be	Atomic weight of Mg	Atomic weight of Ca
8	$\frac{8+40}{2}=24$	40
Atomic weight of Cu	Atomic weight of Ag	Atomic weight of Au
63	$\frac{63+197}{2}=130$ But the mass of Ag is 108	197

The triads do not follow Dobereiner's law of triads is Cu, Ag, Au.

Hence, the correct answer is Option D.