Classification of elements and periodicity of elements

               1. Introduction

The Periodic Table as it stands today and the Modern Periodic Law. We will also learn how the periodic classification follows as a logical consequence of the electronic configuration of atoms. Finally, we shall examine some of the periodic trends in the physical and chemical properties of the elements. 

 WHY DO WE NEED TO CLASSIFY ELEMENTS ?

We know by now that the elements are the basic units of all types of matter. In 1800, only 31 elements were known. By 1865, the number of identified elements had more than doubled to 63. At present 118 elements are known. Of them, the recently discovered elements are man-made. Efforts to synthesise new elements are continuing. With such a large number of elements it is very difficult to study 

individually the chemistry of all these elements and their innumerable compounds individually. To ease out this problem, scientists searched for a systematic way to 

organise their knowledge by classifying the elements. Not only that it would rationalize known chemical facts about elements, but even predict new ones for undertaking further study.

GENESIS OF PERIODIC CLASSIFICATION

Classification of elements into groups and development of Periodic Law and Periodic Table are the consequences of system

the knowledge gained by a number of scientists

through their observations and experiments.

The German chemist, Johann Dob e re in er in

early 1800’s was the first to consider the idea

of trends among properties of elements. By

1829 he noted a similarity among the physical

and chemical properties of several groups of

three elements (Triads). In each case, he noticed that the middle element of each of the Triads had an atomic weight about half way between the atomic weights of the other two. Also the properties of the middle element were in between those of the other two members. Since Dobereiner’s relationship, referred to as the Law of Triads, seemed to work only for a few elements, it was dismissed as coincidence. The next reported attempt to classify elements was made by a French geologist, A.E.B. d e Ch an co ur t o is in 1862. He arranged the then known elements in order of increasing atomic weights and made a cylindrical table of elements to display the periodic recurrence of properties. This also did not attract much attention. The English chemist, John Alexander New lands in 1865 pro founded the Law of Octaves. He arranged the elements in increasing order of their atomic weights and noted that every eighth element had properties similar to the first element .The relationship was just like every 

eighth note that resembles the first in octaves

of music. Newlands’s Law of Octaves seemed

to be true only for elements up to calcium.

Although his idea was not widely accepted at

that time, he, for his work, was later awarded

Davy Medal in 1887 by the Royal Society,

London.

The Periodic Law, as we know it today owes

its development to the Russian chemist, Dmitri

Mendeleev (1834-1907) and the German

chemist, Lot h a r Meyer (1830-1895). Working 

independently, both the chemists in 1869

proposed that on arranging elements in the

increasing order of their atomic weights,

similarities appear in physical and chemical

properties at regular intervals. L o t h  a r Meyer plotted the physical properties such as atomic volume, melting point and boiling point

against atomic weight and obtained a

periodically repeated pattern. Unlike New lands, Lot h a r Meyer observed a change in

length of that repeating pattern. By 1868,

Lot h a r Meyer had developed a table of the

elements that closely resembles the Modern 

Periodic Table. However, his work was not

published until after the work of Dmitri

Mendeleev, the scientist who is generally

credited with the development of the Modern

Periodic Table.