Question

State two reasons for rejecting Triads classification of elements.

Answer 1

Two reasons for rejecting Triads classification of elements:

1. This law was applicable only to three groups of elements and Dobereiner was not able to classify all the known elements during his time into triads.
2. This law was neither very helpful in the prediction of number nor the properties of the undiscovered elements.

Dobereiner's Law of Triads:

• Dobereiner arranged the elements in a group of three (tri meaning three) elements.
• The group of three elements was made such that the atomic mass of the middle element of the triad was approximately equal to the arithmetic mean (average) of the atomic masses of the other two elements of that triad when arranged in their increasing order of atomic mass.
• Example: Triad of Lithium, Sodium and Potassium:

Elements of the triad arranged in the increasing atomic mass	Atomic mass (u)
Lithium (Li)	6.9
Sodium (Na)	23.0
Potassium (K)	39.0

The mass of the middle element of the triad i.e. Sodium= $\frac{AtomicmassofLithium+AtomicmassofPotassium}{2}$

$AtomicmassofSodium=\frac{6.9+39.0}{2}=22.95=23u$