Mendeleev arranged the known elements in order of their increasing atomic weights, grouping together elements with similar properties and leaving out blank spaces wherever necessary. He also made the bold prediction that there were some unknown elements which, of course, would be discovered in due course of time and would fill these blank spaces. He even predicted their properties in the light of the properties of the other elements in the same group. Later on, these unknown components were discovered and were found to possess precisely the same properties as expected by chemist. For example, both gallium and germanium were not known at the time Mendeleev proposed his periodic table.Mendeleev named these components as Eka-aluminium and ekasilicon as a result of he believed that these would be kind of like aluminium and semiconductor severally . These elements were discovered later and Mendeleev' s predictions were found to be remarkably correct.
Characteristics of Mendeleev 's Periodic Table
Inert gases were not known when the Mendeleev periodic table was formulated. With the discovery of inertgases, a new group called zero group was added to the Mendeleev's original periodic table. It is evident that modified Mendeleev's periodic table consists of:
(a) Nine vertical columns called groups
These are designated as I, II, III, IV, V, VI, VII, VIII and Zero.Except for the teams VIII and nil, every cluster is additional divided into 2 sub-groups selected as A and B.The elements that lie on the paw facet of every cluster represent sub-group a jiffy those placed on the proper hand facet kind sub-group B.This sub-division is formed on the idea of the distinction in their properties . Group VIII contains 9 components in 3 sets every containing 3 components . Group zero consists of inert gases.
(b) Seven horizontal rows called periods
These are numbered from 1 to 7. First period contains only 2 elements and is the shortest period. Second and third periods contain 8 elements each. These are also called short periods.Fourth and fifth periods contain eighteen components every and ar referred to as long periods, Sixth amount contains thirty two components and thus is termed the longest amount, Sixth period contains 32 elements and hence is called the longest period. Seventh period is, however, incomplete and contains only 24 elements.
Importance of Mendeleev's Periodic Table
Some important contributions of the Mendeleev's periodic table are listed below
Systematic Study of the Elements
The Mendeleev's periodic table simplified and systematized the study of the elements and their compounds since their properties could not be studied as groups or families rather than individuals. Thus, knowing the properties of one element in a group, the properties of the other elements in the group can be easily predicted.
Prediction of New Elements
At the time when Mendeleev's periodic table was formulated, only 56 elements were known. Therefore, while arranging the elements according to their properties, Mendeleev left some blank spaces or gaps. These gaps represented unknown elements. Further, Mendeleev predicted the properties of these unknown elements on the basis of their positions. For example, he predicted the properties of gallium (eka aluminium) and germanium (eka silicon) which were unknown at that time. Later when these elements were discovered, their properties were found to be similar to those predicted by Mendeleev.
Correction of Doubtful Atomic Weights
Mendeleev's periodic table has helped in correcting the doubtful atomic weights of some elements. For example, Beryllium was assigned an atomic weight of 13.5 on the basis of its equivalent weight (4.5) and valency (wrongly calculated as 3). If beryllium has an atomic weight of 13.5, it should have been placed between Carbon (atomic weight = 12) and Nitrogen (atomic weight = 14). But no covalent place was available in between C and N and furthermore properties of beryllium did not justify such a position. Therefore, valency 2 was assigned to beryllium which gave it an atomic weight of 4.5 x 2 = 9 and it is was placed at its proper position between lithium (atomic weight = 7) and boron (atomic weight = 11)
Defects in the Mendeleev's Periodic Table
In spite of many advantages, Mendeleev's periodic table suffers from many series defects. Some of these are discussed below.
1. Anomalous or controversial position of hydrogen: Hydrogen is placed in group IA. However it resembles the elements of both the group IA (alkali metals) and group VIIA (halogens). Therefore, the position of hydrogen in the periodic table is controversial.
2. Anomalous pairs of elements: Some elements with higher atomic weight precede the elements with lower atomic weight. For example, Ar (at weight = 39.9) precedes Potassium (at. weight=39.1), Co (at. weight=58.9) precedes Ni (at. weight = 58.7) and Te (at. weight = 127.6) precedes lodine (at. weight =126.9). No justification has been provided for this ambiguity.
3. Position of isotopes: Isotopes are the atoms of the same element having different atomic weights.Therefore, according to Mendeleev's classification, these should be placed at different places in the periodic table according to their atomic weights. For example, Isotopes of Hydrogen i.e., Protium , Deuterium and Tritium with atomic weights 1, 2 and 3 should be placed at different places. However, isotopes of an element have not been given different places in the periodic table.
4. Some dissimilar elements are groups together while some similar elements are placed in different groups: For example, alkali metals such as Li, Na, K etc. (group IA) are grouped together with coinage metals such as Cu, Ag and Au (group IB) though their properties are quite different. At the same time, certain chemically similar elements like Cu (group IB) and Hg (group IIB) have been placed in different groups.
5. Position of lanthanides and actinides: The 14 elements following lanthanum from atomic numbers 58 to 71 (commonly known as lanthanides) and the 14 elements following actinium from atom number 90 to 103 (commonly known as actinides) have not been given proper positions in the main frame of the periodic table but have been placed in two separate rows at the bottom of the periodic table.
6. Position of elements of group VIII: No proper place has been allotted to nine elements of group VIII which have been arranged in three triads without any justification.
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