Mendeleev's(or mendeleef 's) periodic table:
"The physical and chemical properties of the elements are a periodic function of their atomic masses.The statement is generally known as mendeleev's periodic law and the table proposed on this basis is known as mendeleev's periodic table.In simple words,the law conveys that when the elements are arranged in order of their increasing atomic masses,elements with similar properties are repeated after certain intervals or periodicity. The discovery of the law helped chemists to classify the elements in an orderly way and properties of certain undiscovered elements could also be predicted.Two elements were assigned with properties even before their discovery.These were named eka-silicon (similar to silicone) and eka-aluminium (similar to aluminium). When later on these elements were found and their properties were analysed there was a marked similarity in these properties with those assigned by mendeleev.His periodic table helped in correcting from 13.5 to 9.
Thought the periodic table was modified at various stages to accommodate new elements,yet it retains its old name and is known as modified mendeleev's periodic table.
Characteristics of mendeleev's periodic table:
The modified from of mendeleev's periodic table consists of seven (7) horizontal rows called periods and nine (9)vertical columns called groups.
(A) Periods:There are seven periods in all.These are numbered from I to VIII and zero.
(1) First period contains only two elements and is the shortest period.
(2) Second and third period,contain 8 elements each. These periods are also called short periods.
(3) Fourth and fifth period,contain 18 elements each.These periods are also called long periods.
(4) Sixth period contains 32 element and is the longest period.This also includes 14 elements of rare earth series.
(5) Seventh period is incomplete and contains only 24 element.
(B) Groups:There are nine groups in all.These are numbered from I to VIII and zero.
(1) The zero group element,also known as inert gases,were not known at that time.
(2) Groups I to VII are known as normal element and each one of them is further divided into sub-groups called sub-group 'a' and sub-group 'b'.
(3) The elements which lie on the left hand side of each group constitute sub-group 'a' while those placed on the right hand side from sub-group 'b'.
(4) Group VIII consists of nine elements in three sets,each containing three elements.
Importance of mendeleev's periodic table:
Some importance uses of mendeleev's periodic table are as follows:
1. It simplified and systematized the study of the elements and their compounds since their properties could be studied as groups or families rather than individuals.
2.It has helped in correcting the doubtful atomic weights of some elements.
3.It provided a guide for the discovery of new elements for which mendeleev had left gaps in the table.He has predicted properties of germanium,discovered later on and found of same predicted properties.
Defects in the mendeleev's periodic table:
Inspite of its useful role in the study of chemistry,mendeleev's table had many drawbacks.Some of these are as follows :
1.The position of hydrogen in the periodic table is not certain because it can be placed in group i-a as well as in group VIII-a.This is due to the reason that as it resembles both alkali metals of group i-a and the halogens of group VII-a.
2.Isotopes have no separate position in the periodic table.
3.No satisfactory position was provided for lanthanoides and actinoides,rather they have been placed in two rows at the bottom of the table.
4.No attempt has been made to separate metals from non-metals,active metals from inactive metals and normal elements from transition elements.
5.The position of certain pair of elements were inverted and they appear as misfits in the periodic table e.g.,cobalt comes before nickel.
6.No separate place has been allotted to nine element of group VIII which have been arranged in three triads without any justification.
7.Mendeleev did not explain the cause of periodicity of elements.