Dmitri Ivanovich Mendeleev, a Russian chemist, in 1869, delivered a remarkable contribution to arrange chemical elements in the Periodic table. He studied the chemical properties of a large number of the elements and stated that the chemical properties of the elements are the periodic function of their atomic masses.The work was published in Zeitschrift für Chemie. The law became popular as Mendeleev's Periodic Law. Later, the periodic law was modified with the following statement.The physical and chemical properties of the elements are the periodic functions of their atomic masses.The main points of periodic law as stated by Mendeleev in his original paper are:
1) The elements, if arranged according to their atomic masses, exhibit an apparent periodicity of properties.
2) Elements which are similar as regards to their chemical properties have atomic masses which are either of nearly same value (e.g. Pt, Ir, Os) or which increase regularly (e.g. K, Rb, Cs).
3) The arrangement of the elements, or of groups of elements in the order of their atomic masses, corresponds to their so-called valencies, as well as to their distinctive chemical properties.
4) The elements which are most widely distributed in nature have small atomic masses and possess sharply defined properties.
5) The magnitude of atomic mass determines the character of the element.
6) Many yet unknown elements may be discovered.
7) The atomic mass of an element may sometimes be corrected with the aid of the knowledge of the atomic masses of the adjacent elements.
8) Certain characteristic properties of elements can be foretold from their atomic masses.
The original table, however, has undergone many modifications with the discovery of new elements.
a) In the periodic table, the elements are arranged in vertical columns called groups and horizontal rows are known as periods.
b) There are seven periods or horizontal rows in Mendeleev's periodic table. In order to accommodate more elements the periods 4, 5, 6 and 7 are divided into two halves. (As shown in above figure, the first half of the elements are placed in the upper left corners and the second half of them occupy the lower right corners in each box).
c) There are eight groups (noble gas was not introduced in Mendeleev's time which was introduced later as zero group, giving the total of nine groups) designated by the Roman numbers from I to VIII. Group VIII contains traits of elements. Later, in modified form of periodic table, each group except VIII has been divided into two sub-groups.
d) Zero-group: Mendeleev arranged only about 60 elements known at that time. After the discovery of the inert gas elements by William Ramsay and Lord Rayleigh, these were included in the zero group. Ramsay and Rayleigh were awarded Nobel prize in 1904 for their discoveries.
The formulation of periodic law by Mendeleev revolutionized the systematic study of chemistry. The scientific benefits of the periodic table are listed below:
When the periodic table was offered for the first time, the systematic classification of the chemical elements shortenedtheir studies into the study of nine groups (zero group included) and seven groups. Thus, by knowing the properties of an element in a particular group, the properties of the rest of the elements in the group can be predicted.
Mendeleev laid more stress on similarity in properties rather than on increasing atomic mass of the elements. So, whenever a particular element did not fit in the arrangement, he left a gap in the periodic table. Thus, many gaps for the undiscovered elements were left in the periodic table by Mendeleev. However, he predicted the properties of the undiscovered elements by considering the properties of adjacent elements. This helped to a great extent in the discovery of these elements at a later stage.
For instance, the elements scandium (Sc), gallium (Ga), and germanium (Ge) were not known at Mendeleev's time, but he had predicted their existence in advance of their discovery. Later, the properties of 'Eka- boron' and 'Eka- silicon' predicted on the basis of periodic law were found to exactly match the properties of Scandium and Gallium after those elements were discovered. The properties of Eka- aluminium(predicted) and gallium (named later) and Eka-silicon (predicted) and Germanium (named later) are given for reference in the table below.
|Melting point (K)||low||29.78||high||1231|
|Formula of oxide||E2O3||GaO3||EO2||GeO2|
|Formula of chloride||ECl3||GaCl3||ECl4||GeCl4|
Mendeleev's periodic table helped in correcting the atomic masses of some of the elements, based on their positions in the periodic table. For example, the atomic mass of Beryllium was corrected from 13.5 to 9. Atomic masses of indium, gold, and platinum were also corrected.
In spite of its great usefulness, the periodic table suffered from certain serious defects and anomalies. The major shortcomings of the tables are :
A correct position for hydrogen could not be assigned in the periodic table. Hydrogen could be placed in the alkali metals group (group IA) as well as in the halogens group (group VIIA). It is still not certain whether to place H-atom in group IA or VIIA.
Although the elements on Mendeleev's periodic table had been arranged in order of increasing atomic masses, in some cases, the element with higher atomic mass preceded the element with lower atomic mass. Some examples are-
I) Argon (39.9) is placed before potassium (39.1).
II) Cobalt (59.9) is placed before nickel (58.6).
III) Tellurium (127.6) is placed before iodine (126.9).
IV) Thorium (232) is placed before protactinium (231).
There was no place for isotopes of various elements, which were discovered later. If we arrange the elements in order of their increasing atomic masses, then, it becomes difficult to assign the proper place for a large number of isotopes. For example, hydrogen has 3 isotopes but is assigned only one position.
Some similar elements were separated and dissimilar elements were grouped together. For instance, the pairs such as copper and mercury, silver and thallium, barium and lead have similar properties but these were placed in different groups. On the other hand, elements of group IA such as lithium, sodium, and potassium were grouped with dissimilar elements such as copper, silver, and gold.
Fourteen elements that follow lanthanum are called lanthanides and fourteen elements that follow actinium are called actinides. These were not given proper places in Mendeleev's periodic table.
No attempt had been made to place metals and non-metals separately in the periodic table. Most of the groups were mixed groups consisting of metals, nonmetals, and metalloids.
No proper explanation has been offered for the fact why the elements placed in a group show resemblance in their properties.
Original Mendeleev's periodic table did not indicate any of the noble gases, which were discovered later. These were added by Sir Willian Ramsay as Group Zero, without the disturbance to the basic concept of the periodic table.
Adhikari, Rameshwar; Khanal, Santosh; Subba , Bimala; Adhikari, Santosh; Khatiwada, Shankar Pd. Universal Chemistry XI. First. Vol. 1st. Kathmandu: Oasis Publication, 2069.
Chaudhary, Ganga Ram; Karna, Shila Kant Lal; Sharma, Kanchan; Singh, Sanjay; Gupta, Dipak Kumar. A Textbook of Higher Secondary Chemistry XI. Ed. 2nd. Kathmandu: Vidyarthi Pustak Bhandar, 2069 (2012).