Question

Identify the set of atomic numbers which does not obey the Dobereiner's Law of triads.

• A. $20,38,56$
• B. $3,11,19$
• C. $17,35,53$
• D. $5,13,31$

Answer 1

The correct option is D

$5,13,31$

Explanation for the correct option: (d)

1. According to Dobereiner's law of triads, when the elements are arranged in the increasing order of atomic masses the atomic mass of the central element will be approximately equal to the arithmetic mean of the atomic masses of the other two elements.
2. Here in the given atomic numbers $5,13,31$, the atomic number $13$ is not found to be the arithmetic mean of $5$ and $31$.
3. The arithmetic means of $5$ and $31$ is: $\frac{5+31}{2}=18$

Explanation for the incorrect options: (a)

1. The given set of atomic numbers forms Dobereiner's law of triads because the atomic number of the central element is equal to the arithmetic mean of the atomic number of the other two elements.
2. The arithmetic means of $20$ and $56$ is $38$.

(b)

1. The given set of atomic numbers obeys Dobereiner's law of triads.
2. The arithmetic means of $19$ and $3$ is: $\frac{19+3}{2}=11$

(c)

1. The given set of atomic numbers also obeys Dobereiner's law of triads.
2. The arithmetic means of $53$ and $17$ is $35$.

Hence the correct option is (d) and the given set of atomic numbers does not obey Dobereiner's law of triads.