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

Explanation for Correct option:

D) Cu(63.5),Ag(107.8),Au(196.9)

Average of atomic masses of Cu and Au is not equal to atomic mass of Ag.

$\frac{63.5+196.9}{2}=\frac{260.4}{2}=130.2$ which is not equal to mass of Ag.

Explanation for Incorrect options:

A)Li, Na, K- Average of atomic mass of Li and K gives the mass of Na.

$\frac{7+39}{2}=\frac{46}{2}=23$

B)Ca, Sr, Ba - Average of atomic mass of Ca and Ba gives the mass of Sr.

$\frac{40+137.2}{2}=\frac{177.2}{2}=88$

C)Be, Mg, Ca - Average of atomic mass of Be and Ca gives the mass of Mg.

$\frac{9+40}{2}=\frac{49}{2}=24.5$

Hence, D) is the correct option- Cu, Ag, Au do not form a Dobereiner triad.