What is meant by Dobereiner’s triads? Explain. - Vidyadip

Question

What is meant by Dobereiner’s triads? Explain.

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Answer

Johann Wolfgang Dobereiner, a German chemist tried to arrange the elements with similar properties into groups.
He identified some groups having three elements each.
These groups of three elements were called triads.
Law of triads:
When the three elements in a triad were arranged in the order of increasing atomic masses, the atomic mass of the middle element is roughly the average of the atomic masses of the other two elements.
For example, lithium $(Li)$, sodium $(Na)$ and potassium $(K)$ constitute a triad.
The atomic masses of $Li,$ Na and $K$ are $6.9 u$, $23.0 u$ and $39.0 u$ respectively.
The atomic mass of middle element, sodium $(Na)$ is roughly the average of the atomic masses of lithium $(Li)$ and potassium $(K).$

Elements	Symbol	Atomic mass
Lithium	$Li$	$6.9 \ u$
Sodium	$Na$	$23.0 \ u$
Potassium	$K$	$39.0 \ u$

Average of atomic masses of
$Li$ and $K =\frac{6.9+39}{2}=22.95 \cong 23.0 u$
Dobereiner's triads are as follows:

Table 1: Dobereiner’s triads

$Li$	$Ca$	$Cl$
$Na$	$Sr$	$Br$
$K$	$Ba$	$I$

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