what are the limitations of mendeleevs periodic table
Hi Snoby.. !!!!
Limitations are :
Position of hydrogen : Hydrogen is placed in Group IA. However, it actually resembles the elements of Group-IA (alkali metals ) as well as the elements of Group VII-A (halogens ). Thus, the position of hydrogen in the periodic table is not clear.
Position of isotopes : On the basis of atomic weight , various isotopes of the same elements should be assigned different places in the periodic table. Mendeleev could not provide separate places for isotopes.
Position of lanthanides and actinides : Fourteen elements following Lanthanum(known as lanthanides or rare earths) and the fourteen elements following Actinium (known as actinides or transuranic elements ) have not been provided separate and proper places in the Mendellev's table, rather they have been placed in two rows at the bottom of the table.
Disimilar elements placed together : Noble metals like Cu, Ag and Au are placed along with chemically dissimilar alkali metals in Group I . Similarly, Mn possessing very few similarities with halogens have been placed in VII group.
Similar elements separated : In Mendeleev's periodic table, certain chemically similar elements such as copper and mercury ; gold and platinum have been placed in different groups.
Anomalous pairs : In the Mendeleev's Table based on atomic weight, the positions of certain pairs , e.g. Argon( at. wt = 39.94) and potassium ( at. wt = 39.1) : Cobalt( at wt =58.93 ) and nickel ( at wt = 58.71 ) ; Tellurium at wt = 127.60) and iodine (atomic weight = 126.90 ) would be reversed. In other words, certain pairs of elements are misfit in the periodic table, if atomic weight is the basis of classification.
Hope this helps !!!!!!
Limitations of Mendeleevs periodic table:
The position of hydrogen is not justified in Mendeleevs periodic table. He positioned it in the first column above alkali metals. He did so because hydrogen and alkali metals have similar electronic configurations. Also, hydrogen reacts with halogens, oxygen and sulphur to form compounds whose formulae are similar to those of alkali metals.
Table: Hydrogen and alkali metals reacting with halogens
However, like halogens, hydrogen too is a gas and exists as a diatomic molecule (H2). It forms covalent compounds like halogens, and unlike alkali metals. Hence, it can also be placed above the halogen group.
Hence, we can say that hydrogen resembles alkali metals as well as halogens, and can be placed above either of them. This means that hydrogen cannot have a fixed position in the periodic table. Therefore, it is a limitation of Mendeleevs periodic table that hydrogen is not assigned a correct position in it.
The discovery of isotopes revealed another limitation of Mendeleevs periodic table. Atoms of the same elements having different number of neutrons are called isotopes. Isotopes have the same number of protons and electrons, but different number of neutrons. For example, the isotopes of chlorine are Cl-35 and Cl-37. They have the same chemical properties, but different atomic masses. Since Mendeleevs periodic table is based on the atomic masses of elements, isotopes have to be placed in different columns despite the fact that they represent the same element.
In Mendeleevs periodic table, atomic masses do not increase in a regular manner. As a result, it is not possible to predict the number of elements that might be present between two elements.
Although Mendeleev arranged the elements in the increasing order of their atomic masses, there are instances where he has placed an element with a slightly higher atomic mass before an element with a slightly lower atomic mass. For example, cobalt (whose atomic mass is 58.9) is placed before nickel (whose atomic mass is 58.7). This reversal was done to maintain a consistency in the properties of the elements present in a column, i.e., to group the elements with similar properties together.
Refer 10th Science Text book Page No.84. - LIMITATIONS OF MENDELEEV' S CLASSIFICATIONS
position of lanthanide tell ma about this