He examined the relationship between atomic weights of the elements and their physical and chemical properties. Among chemical properties, Mendeleev mainly concentrated on the compounds formed by the elements with hydrogen and oxygen because they are highly reactive and hence formed compounds with almost all the elements.
The formulae of the hydrides and oxides formed by the various elements was made the basis of classification of elements.
Mendeleev in 1869, proposed periodic law.
It states that
The physical and chemical properties of elements are a periodic function of their atomic weights i.e. when the elements are arranged in order of their increasing atomic weight, elements with similar properties are repeated after certain regular intervals. This repetition of properties of elements after certain regular intervals is called periodicity of properties.
Characteristics of Mendeleev’s periodic table
1)Mendeleev’s arranged the then known elements in order of their increasing atomic weights, grouping together elements with similar properties and leaving out blank space wherever necessary.
2)He also made the prediction that there were some unknown elements which, would be discovered in due course of time and would fill these blank spaces.
3)He predicted their properties in the light of the properties of the other elements in the same group. Later on, these unknown elements were discovered and were found to possess exactly the same properties as predicted by Mendeleev.
4)Noble gases were not known at the time of Mendeleev. When these gases were discovered, a new group called the zero group was added to Mendeleev original periodic table.
5)Two group of 14 elements each, called lanthanides and actinides were placed at the bottom of the periodic table.
6)Mendeleev’s periodic table consists of :
a) 9 vertical columns called groups. These are designated as 0, , , , ,, , ,
b) Except for group 0 and ,each group is further divided into two subgroup designated as A and B.The elements which lie on the left hand side of each group constitutes sub-group A while those placed on the right hand side from sub-group B. This subdivision is made on the basis of the differences in their properties.
c) Group contains 9 elements in three sets each containing three elements. Group 0 has no subgroup. It consists of only one vertical column of inert gases.
d) 7 horizontal rows called periods. These are numbered from 1 to 7.
Significance of Modified Mendeleev’s Periodic Table
1) Systematic study of the elements :The Mendeleev’s periodic table simplified and systematised the study of the elements and their compounds since their properties could now be studied as groups or families rather than individual. Knowing the properties of one element in a groups, the properties of the other elements in the group can be easily predicted.
2) Prediction of new elements: While arranging the elements according to their properties ,Mendeleev left some blank spaces or gaps. these gaps represented unknown elements. Mendeleev predicted the properties of these unknown element on the basis of their positions. When these elements were discovered ,their properties were found to be similar to those predicted by Mendeleev.
3) Correction of doubtful atomic weights: Mendeleev periodic table has helped in correcting the doubtful atomic weights of some elements.
Defects in the Mendeleev’s Periodic Table
1)Anomalous position of hydrogen: Hydrogen is placed in group A .It resembles the elements of both group A and group A .The position of hydrogen in periodic table is not fixed but is anomalous.
2)Position of isotopes: According to Mendeleev’s classification, isotopes should be placed at different places in the periodic table according to the atomic weights. However, isotopes of an element have not been given different places in the periodic table.
3)Some dissimilar elements are grouped together while some similar elements are placed in different groups: Alkali metals such as Li, Na, K, are grouped together with coinage metals such as Cu, Ag, Au though their properties are quite different. Certain chemically similar elements like Cu an Hg have been placed in different group.
4) Anomalous pairs of elements: Some elements with higher atomic weights precede the elements with lower atomic weights.
5)Position of elements of group : No proper place has been allotted to 9 elements of the group which have been arranged in 3 trials in 4th, 5th and 6th period without any justification.
6)Position of lanthanides and actinides :The 14 elements following lanthanum from atomic number 58- 71 commonly known as lanthanides are lanthanoids and another group of 14 elements from atomic number 90-103 known as actinides are actinoids have not been given proper position in the periodic table but have place in 2 separate rows at the bottom of the periodic table.
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