Question

What is the difference between the Mendeleev’s Periodic Law and the Modern Periodic Law?

Answer 1

Hint: In the Mendeleev’s Periodic Table, elements were arranged on the basis of their atomic masses while in the Modern Periodic Table, elements were arranged on the basis of their atomic numbers.

Complete answer:
Mendeleev interpreted a relationship between the elements. He stated that the properties such as the chemical and physical properties of the elements are a periodic function of their atomic masses. While arranging the elements on the basis of atomic masses certain discrepancies were found.
It had certain drawbacks. Moseley showed that the elements could be arranged on the basis of their atomic numbers in a periodic table. The Modern Periodic Law states that the properties such as the chemical and physical properties of the elements are a periodic function of their atomic numbers. The major differences between the Mendeleev’s Periodic Law and the Modern Periodic Law are:

MENDELEEV’S PERIODIC LAW	MODERN PERIODIC LAW
The properties such as the chemical and physical properties are said to be the periodic function of their atomic masses.	The properties such as the chemical and physical properties are said to be the periodic function of their atomic numbers.
The classification will be based on the formulas of hydrides and oxides.	The classification will be based on the electronic configuration of the elements.
Noble elements were not included in the periodic table.	Noble elements were included in the periodic table.
In this periodic table, the elements with larger atomic masses are placed before the elements with smaller atomic masses.	The elements are arranged according to the increasing order of their atomic number and atomic masses.
Due to different atomic weight, isotopes are placed in different positions.	Due to same atomic number, the isotopes are placed in same position
In Group VIII, the elements such as Fe, Co, Ni are placed together.	But in this periodic table, the elements such as Fe, Co, Ni are placed in the d-block.

Some drawbacks of Mendeleev’s Periodic Table were:
No reasons were given for the triad elements of VIII group. Such special groupings are not given in the modern table.
In the Mendeleev’s periodic table, there were 63 elements which were arranged as only 63 were known at that time.
Atomic number is found to be a fundamental property differentiating each element and therefore has a better basic nature to decide the physical and chemical properties of elemental atoms than atomic weights.

Note:
This is to be noted that the main difference between the two laws is based on how they classify elements. Mendeleev’s Law classifies elements based on their atomic weights while the Modern Periodic Law classifies the elements based on their atomic number.