Question

Write short notes on:
a. Mendeleev’s periodic law.
c. Position of isotopes in the Mendeleev’s and the modern periodic table.

Answer 1

a. Mendeleev's periodic law, "The properties of elements are periodic function of their atomic masses."
The periodic table was organised by Mendeleev according to the chemical and physical properties of the elements. These were the molecular formula of the hydrides and oxides of the elements, melting points, boiling points and densities of the elements and their hydrides and oxides. Mendeleev determined that the elements with similar physical and chemical properties repeat after a definite interval. So, as per this finding he stated the following periodic law that properties of elements are periodic functions of their atomic masses. The vertical columns in the Mendeleev’s periodic table are the groups while the horizontal rows are known as periods.

b.Isotopes: Isotopes are the atoms having same atomic number but different atomic masses.
Therefore, according to Mendeleev’s classification these should be placed at different places depending upon their atomic masses.
For example, hydrogen isotopes with atomic masses 1,2 and 3 should be placed at three places. However, isotopes have not been given separate places in the periodic table because of their similar properties. So this was drawback of Mendeleev's periodic table as he could not explained the position of isotopes.
Modern periodic table is based upon arrangement of the elements on the basis of their atomic number. So that, all the isotopes of hydrogen should be placed at same place depending upon their atomic number.