Question

Explain the laws of Dobereiner and Mendeleev for classification of elements?

Answer 1

Dobereiner's law of triads states that if three elements are arranged in the increasing order of their atomic masses, the atomic mass of intermediate (middle) element is similar to the average of the atomic masses of the first and the third element.

Example - In a triad of Lithium $\left(Li\right)$, sodium $\left(Na\right)$ and potassium $\left(K\right)$, atomic masses of $Li$ is $7$ and $K$ is $39$. The atomic mass of $Na$ is $23$

$(\frac{7+33}{2}=23)$; thus proving Dobereiner law of triads.

Mendeleev's periodic law states that the properties of the elements are the periodic function of their atomic masses. When Mendeleev arranged the elements in increasing order of their atomic masses in seven groups, he found that the elements falling in the same group have similar properties.

Example - $Li,Be,B,C,N,O$ and $F$ placed in the second period of Mendeleev's periodic table showed similar properties.