Dmitri Mendeleev

Dmitri Mendeleev was a Russian Chemist born in 1834 at Tobolsk in Siberia. He Studied Science in St. Petersburg. In 1856 he got his graduation. He became professor in Saint Petersburg Technological Institute and Saint Petersburg State University in 1864 and 1865.

Classification of Elements by Mendeleev

• In order to classify elements, Mendeleev considered atomic masses as fundamental property. Based on atomic mass Mendeleev compared elements based on their atomic masses. He also studied their physical and chemical properties. While studying chemical properties he primarily considered the compounds formed by elements with oxygen and hydrogen as these elements forms compounds with most of the elements.
• After his study he started arranging the elements based on increasing atomic masses. With the physical and chemical properties he found that every eighth element has properties similar to first. So he placed every eighth element below the first. likewise he arranged all 63 elements in rows and columns.

Mendeleev’s Law

Mendeleev’s Law is given as

“The physical and chemical properties of elements are a periodic function of their atomic masses”.

The tabular arrangement of elements based on Mendeleev’s law is called Mendeleev’s Periodic table.

Mendeleev’s Periodic Table

Features of Mendeleev’s Periodic Table

• In Mendeleev’s Periodic table, the horizontal rows are called periods and vertical columns are called groups.
• There are seven periods and eight groups in Mendeleev’s periodic table. Each group from I to VII are divided into two subgroups A and B.





• In each period elements are placed based on increasing atomic masses.
• In any period, the properties of elements show regular gradation. E.g. Metallic character decreases, while non-metallic character increases.

Merits of Mendeleev’s Periodic Table

• The periodic table succeeded in classifying all elements known at that time.
• The vacant places present in the periodic table correctly predicted the existence of undiscovered element. Based on their position it become possible to predict the properties of undiscovered elements before their actual discovery. E.g. Predicted Elements were Eka-boron, Eka-aluminium, Eka-silicon. After discovery these elements are named Scandium, Gallium and Germanium respectively. The properties of predicted element and the discovered element were almost same. E.g. Predicted atomic weight of Eka-aluminium was 68 and the discovered element Gallium had atomic weight 69.72.
• In Mendeleev’s periodic table, noble gases are placed in separate group without disturbing the position of already placed elements.

Demerits of Mendeleev’s Periodic Table

• Position of Hydrogen: Hydrogen not only shows properties similar to alkali metals but also shows properties similar to Halogens as well. E.g. Hydrogen form compound H₂O which is exactly similar to Li₂O, Na₂O, K₂O but the electronic configuration of hydrogen suggests deficiency of one electron which the the characteristic property of halogens. And so, Mendeleev’s periodic table failed to classify hydrogen.
• Classification of Isotopes: Elements having same atomic number but different Mass Number are called Isotopes. Since isotopes have different masses hence there should be dedicated position to each isotope in Mendeleev’s periodic table. On the other hand, the chemical properties of isotopes are similar hence they must be placed at Same Position. The ambiguity posed a question on position of isotope in periodic table.
• Arrangement on the Basis of Atomic Mass: At some places elements with higher atomic mass was placed before the element with lower atomic mass. e.g. Cobalt with atomic mass 58.93 was placed before Nickel with atomic mass 58.71 and so the classification no longer remained orderly arrangement of atomic masses.
• Properties in a Group: Ideally in a group the properties of element must be same but in the periodic table elements with different properties placed in the same group. e.g. Manganese is a metal which is placed in the group of nonmetals i.e. halogens.

Keywords: Merits of Mendeleev’s Periodic Table, Demerits of Mendeleev’s Periodic Table, Classification of Isotopes, Position of hydrogen

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