Introduction of Periodic Classification of Elements in Chemistry

A systematic study of elements and their compounds is possible only when these elements are arranged in such a way that the arrangement is closely related to their properties and makes the study simpler, this systematic classification of elements can be done with the help of periodic table.

The Need of Classification of Elements and the Periodic Table:

Very few elements were known till the end of the 17th century and the discovery of elements gained a big momentum in the 18th and 19th centuries where several new elements were also discovered. With this discovery the chemists began to realise that these elements should be arranged in a proper way to facilitate their systematic study. Several efforts were made to classify the known elements in a systematic way. The first meaningful effort in this direction was made by Dmitri Mendeleev, Russian Chemist in 1869 who deeply studied the physical and chemical properties of the elements and classified them on the basis of a law known as Periodic Law. The arrangement of the elements on the basis of this law was given the name Periodic Table.

Definition of Periodic Table:

A periodic table is defined as an arrangement of all the known elements based on a periodic law in a manner that the elements with similar properties fall in the same vertical column called a group.

Earlier Developments in the Classification of Elements:

Before Mendeleev proposed his Periodic Table, several efforts were made to classify the known elements in a systematic way. Some of these are:-

• Unitary Theory: William Prout in 1815, suggested the element’s atomic masses or weights values were either whole number or varied slightly from that whole number and is applicable for all the elements which were made up of hydrogen atoms.

• Dobereiner’s Triads:- Dobereiner in 1817, a German Chemist suggested that the elements could be arranged in a triad (group or set of 3 elements) in such a manner that the middle one had atomic mass almost equal to the average of the other two elements. Examples; Lithium, Sodium, Potassium and many more.

• Cooke’s Homologous Series:- J.P. Cooke’s in 1854 classified the elements on the basis of their chemical and physical properties in a homologous series. For example, Nitrogen, Phosphorus, Arsenic etc.

• Newland’s Law of Octaves:- An English Chemist John A.R. Newlands in 1865 gave the Law of Octaves. For example; Hydrogen, Lithium, Beryllium, Boron, Carbon etc.

• Lother Meyer’s Arrangement of Elements:- A German Chemist Lother Meyer in 1869, explained several element’s physical properties like atomic volume, melting point, density, thermal conductivity, boiling point etc. against atomic mass and also found that these physical properties varied in a periodic way.

Mendeleev’s Periodic Table

Mendeleev’s Law was given in 1869 by Dmitri I. Mendeleev states that “the chemical and physical properties of the elements with the properties and formulae of their compounds are the periodic functions of their atomic masses (atomic weights)” . This law also explained the dependence of chemical and physical properties of the elements upon their atomic weights is periodic. On the basis of his Periodic law, he arranged all the known elements in the table form which is now called Mendeleev’s Periodic Table which is arranged in horizontal columns (periods) and vertical columns (groups). Some of their defects are as follows:-

• Position of Hydrogen is not explained properly.
• Position of isotopes is not explained properly.
• Variable Valency of elements.
• Anomalous pair of elements.
• Elements with dissimilar properties placed in the same groups etc.

Modern Periodic Table

Mosley in 1913 explained that the most fundamental property of an element is the atomic number and not the atomic mass and is responsible for its chemical and physical behaviour. The Modern Periodic Law states that the chemical and physical properties of an element are the periodic functions of their atomic number and not the atomic mass. Based on this, a new Periodic Table is designed which came to be known as the Modern Periodic Table which is the Long or extended form of the periodic table. It consists of 18 groups with 7 periods and is divided into s,p,d and f blocks. It also contains 14 lanthanides like Ce, Pr, Nd etc. and 14 actinides like Th, Pa, U, Np etc.

Classification of Elements in the Periodic Table

The classification of elements in the periodic table on the basis of their electronic Configuration are as follows:-

• Noble Gases: Example; Helium (He), Neon (Ne), Argon (Ar), Krypton (Kr) etc.

• Normal Elements or Representative Elements: It includes s-block like Hydrogen, Lithium etc. and p-block elements like Boron, Carbon etc.

• Transition Elements: It includes d-block elements like Zinc, Cd, La etc.

• Inner Transition Elements: It includes f-block elements i.e. Lanthanides like Ce, Pr, Nd etc. and Actinides like Th, Pa, U, etc.

Conclusion

The periodic table is the table which consists of all the known elements in a systematic way according to their atomic number. The first periodic table is the Mendeleev’s Periodic Table which consists of 65 elements. Long form of the periodic table is based on the atomic numbers which explains the relationship between the electronic configuration and periodic table.