Question

State Dobereiner's Law of Triads for the classification of elements. Support your answer by two examples.

Answer 1

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.
• This law was proposed by a German chemist called Johann Wolfgang Dobereiner in 1817.

Examples:

• 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= $\frac{AtomicmassofLithium+AtomicmassofPotassium}{2}$

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

• Triad of Calcium, Strontium, and Barium

Elements of the triad arranged in the increasing atomic weight	Atomic weight (u)
Calcium (Ca)	40.1
Strontium (Sr)	87.6
Barium (Ba)	137.3

The mass of the middle element of the triad i.e. Strontium= $\frac{AtomicmassofCalcium+AtomicmassofBarium}{2}$

$AtomicmassofStrontium=\frac{40+137.3}{2}=88.65u$