Mendeleev's Periodic Table

Mendeleev's periodic law: It states, "All the physical and chemical properties of the elements are the periodic function of their increasing atomic weight".

Fig: Dimitri Mendeleev

Atomic weight: It is the sum of number of protons and neutrons present in the nucleus of an atom.

Mendeleev's periodic table:

Merits of Mendeleev's periodic table:

1. Mendeleev's periodic table was the first scientific and systematic classification system that made the study of all the known elements easier and more practicable.

   

2. Many elements were not discovered during the time of Mendeleev, so he knowingly left some gaps for the undiscovered one.

   

3. The atomic weight of some elements like Beryllium, platinum, gold, uranium, etc. appeared to be doubtful in the periodic table before Mendeleev periodic table came in prominence. Mendeleev's periodic table gave the right positions to such elements by studying the properties of them.

4. Mendeleev's periodic table inspired subsequent chemists to discover new elements as he left some gaps for them.

Demerits of Mendeleev's periodic table

1. Position of hydrogen: The position of hydrogen in his periodic table is not properly defined. Sometimes hydrogen loses electron like Group I elements and sometimes it gains electrons like those of group VII. Due to this property of hydrogen, it can be placed either in group I or group VII.
2. Separation of chemically similar elements: Some chemically similar elements have been placed in different groups such as copper and mercury, gold and platinum, silver and thallium, barium and lead.
3. Grouping of chemically dissimilar elements: Certain chemically dissimilar elements have been placed in the same group. For example: Copper, silver and gold have been placed in group I along with highly electropositive metals like lithium, sodium and potassium.
4. Position of isotopes: Isotopes have the same atomic number, similar chemical properties but different atomic masses. Based on the periodic law, an isotope is an element, which must have a separate place in the periodic table. However, there are no places for isotopes in his periodic table.
5. Anomalous pair of element: Certain elements having higher atomic weights have been placed wrongly before those with less atomic weight. For example, Argon (Ar) having higher atomic weight(39.9) has been placed before Potassium (K) with atomic weight (39).

   

6. Position of lanthanides and actinides: The fourteen elements from atomic number 58 (cerium) to 71 (lutetium) are termed as lanthanides (rare earth elements). These element have similar properties with that of lanthanum (Last). The elements from atomic number 90(thorium) to 103 (lawrencium) are termed as actinides (trans-uranium elements). These elements have similar properties with that of actinium. Mendeleev could not arrange their property in the fixed position.
7. Position of iron and platinum group: Three elements such as Fe, Cu and Ni are placed in the group VIII. If these elements are separated, the periodic functions of the Mendeleev's periodic table will lost.
8. Mendeleev's periodic table was not able to explain the atomic properties of the elements like valency, metallic characters, reactivity, ionization potential, etc.