5. Periodic Classification of Elements | Class 10 CBSE | Web Notes | Part 2 | Mendeleev's Periodic Table

 5. PERIODIC CLASSIFICATION OF ELEMENTS

MAKING ORDER OUT OF CHAOS – MENDELÉEV’S PERIODIC TABLE

Dmitri Ivanovich Mendeleev (1834-1907, a Russian chemist) was the most important contributor to the development of Periodic Table.

Mendeleev’s Periodic Table proved to be the unifying principle in chemistry. It was the motivation for the discovery of new elements.

Mendeleev started his work with 63 known elements. He examined relationship between the atomic masses (fundamental property) of the elements and their physical & chemical properties.

Mendeleev concentrated on the compounds (hydrides & oxides) formed by elements with oxygen & hydrogen. He selected these elements as they are very reactive and form compounds with most elements. The formulae of the hydrides & oxides were treated as a basic property.

He wrote down the properties of each element on 63 cards and sorted out them with similar properties. He found that most elements were arranged in the increasing order of atomic masses.

There was a periodic recurrence of elements with similar physical & chemical properties. Thus, he formulated a Periodic Law. It states that ‘the properties of elements are the periodic function of their atomic masses’.

In a few cases, Mendeleev had placed an element with slightly greater atomic mass before an element with slightly lower atomic mass. The sequence was inverted to group the elements with similar properties together. E.g. Cobalt (atomic mass 58.9) appeared before nickel (atomic mass 58.7).

MENDELEEV’S PERIODIC TABLE

Mendeleev’s Periodic Table contains groups (vertical columns) and periods (horizontal rows).

R= elements.

Achievements of Mendeleev’s Periodic Table

Mendeleev left some gaps in Periodic Table and predicted the existence of some elements. 

He named them by prefixing Eka (one) to the name of preceding element in the same group. 

E.g. scandium, gallium & germanium, discovered later, have properties similar to Eka–boron, Eka–aluminium and Eka–silicon, respectively.

Properties of eka–aluminium and gallium

Property	Eka-aluminium	Gallium
Atomic Mass Formula of Oxide Formula of Chloride	68 E2O3 ECl3	69.7 Ga2O3 GaCl3

It proves the correctness and usefulness of Mendeleev’s Periodic Table.

Noble gases like helium (He), neon (Ne) & argon (Ar) were discovered very late because they are very inert and rarely present in the atmosphere. They could be placed in a new group without disturbing the existing order.

Limitations of Mendeleev’s Classification

1.   No fixed position to hydrogen because:

·  Its electronic configuration resembles that of alkali metals. Like alkali metals, hydrogen combines with halogens, oxygen & sulphur to form compounds having similar formulae. E.g.

Compounds of H	HCl	H2O	H2S
Compounds of Na	NaCl	Na2O	Na2S

·  Like halogens, hydrogen exists as diatomic molecules and combines with metals and non-metals to form covalent compounds.

2.  Isotopes of an element have similar chemical properties, but different atomic masses. So they cannot be placed in same slots. E.g. Cl-35 & Cl-37 are isotopes of chlorine.   

3.  Atomic masses do not increase in a regular manner from one element to the next. So it was not possible to predict how many elements could be discovered between two elements especially in case of heavier elements.

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👉 Part 1: Early Attempts of classification
👉 Part 2: Mendeleev's Periodic Table
👉 Part 3: Modern Periodic Table