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 !!!!!!
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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.
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2) the position of isotopes is not shown
3)He used the basis of placeing as atomic masses (from lower to higher)due to which some elements with higher atomic mass are placed before followed by elements with lower atomic masses
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in some cases mendeleev placed some elements according to thier similarties properties not by thier increasing order of thier atomic masses.thus the position of these elements was not justified.
isotopes were not given separate places in the peroidic table.
some similar elements were group separately while some dissimilar elements were grouped together.
mendeleev could not explain the cause of peroidicity in the elements.
the position of lathenide and actinide were not included in this table.
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in some cases mendeleev placed some elements according to thier similarties properties not by thier increasing order of thier atomic masses.thus the position of these elements was not justified.
isotopes were not given separate places in the peroidic table.
some similar elements were group separately while some dissimilar elements were grouped together.
mendeleev could not explain the cause of peroidicity in the elements.
the position of lathenide and actinide were not included in this table.
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Limitations of Mandeleev’s periodic table : Inspite of many advantages, the Mendeleev’s periodic table has certain defects also. Some of these are given below ;
(A) Position of hydrogen : Position of hydrogen in the periodic table is uncertain. It has been placed in 1A group with alkali metals, but certain properties of hydrogen resemble those of halogens. So, it may be placed in the group for halogens as well.
(B) Position of isotopes : Isotopes are the atoms of the same element having different atomic masses. Therefore, according to Mendeleev’s classification these should be placed at different places depending upon their atomic masses. For example, hydrogen isotopes with atomic masses 1,2 and 3 should be placed at three places. However, isotopes have not been given separate places in the periodic table because of their similar properties.
(C) Anomalous pairs of elements “ In certain pair of elements, the increasing order of atomic masses was not obeyed. In these, Mendeleev placed elements according to similarities in their properties and not in increasing order of their atomic masses.
For example :
– The atomic mass of argon is 39.9 and that of potassium 39.1 But argon is placed before potassium in the periodic table.
– Tellurium (at. mass = 127.6 ) is placed before iodine (at. mass = 126.9)
(D) Some similar elements are separated, in the periodic table. For example copper (Cu) and mercury (Hg), barium (Ba) and lead (Pb). On the other hand some dissimilar elements have been placed together in the same group.
(E) Cause of periodicity : Mendeleev could not explain the cause of periodicity among the elements.
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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.
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(A) Position of hydrogen : Position of hydrogen in the periodic table is uncertain. It has been placed in 1A group with alkali metals, but certain properties of hydrogen resemble those of halogens. So, it may be placed in the group for halogens as well.
(B) Position of isotopes : Isotopes are the atoms of the same element having different atomic masses. Therefore, according to Mendeleev?s classification these should be placed at different places depending upon their atomic masses. For example, hydrogen isotopes with atomic masses 1,2 and 3 should be placed at three places. However, isotopes have not been given separate places in the periodic table because of their similar properties.
(C) Anomalous pairs of elements ? In certain pair of elements, the increasing order of atomic masses was not obeyed. In these, Mendeleev placed elements according to similarities in their properties and not in increasing order of their atomic masses.
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- Position of hydrogen still dicey. It is not fixed till now.
- Position of Lanthanides and actinides has not been given inside the main body of periodic table.
- It does not reflect the exact distribution of electrons of some of transition and inner transition elements.
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in mendeleevs periodic table the position of H is unsatisfactory. he did not give a plausible explanation and did not place H along with alkali elements nor the halogens.
position of isotopes:
isotopes of an element have similar chemical properties but different masses. thus according to mendeleevs periodic table isotopes should be placed at different positions based on their atomic masses but it was not so.
similar elements were not placed together:
Cu in group 1B and Hg in group 2B. they show similar properties but they were placed in different groups.
anomalous pair of elements:
in mendeleevs periodic table there are a number of anomalous pair of elements where the elements having lower atomic mass has been placed before the element having higher atomic mass.
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* The position of hydrogen was not justified or hydrogen was not given a correct position.
* Isotopes of elements posed a challenge because of the different atomic masses.
* Atomis masses did not increase in a regular manner since nickel and cobalt were arranged in the same group.
(Nickel has atomic mass 28 but cobalt has atomis mass 27)
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2.It is based upon more fundamental property of an element i.e. atomic number.
3.It helped in the correction of wrong atomic number.
4.Some gaps were left in the periodic table for undiscovered elements, but predicted the properties of these element.
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Hydrogen: he did not give a proper place for hydrogen. It resembles with alkali metals as well as halogens.
Isotopes :the atomic mass of isotopes is different but mendeleeves could not give place for isotops.
Inversion of periodic table: he arranged elements in increasing atomic mass but he place cobalt before nikle .
Noble gas :in his periodic table there is no place for noble gases.
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1) Systematic study of elements:-
?As the elements having same properties are grouped together so it becomes very useful to remember the properties of large number of elements in a systematic way.
2) ?Prediction of new elements:-
?Mendeleev predicted the properties of those missing elements from the known properties of the other elements in the same group. On the basis of this, he named Gallium as Eka-Almunium, Germanium as Eka-Silicon and Scandium as Eka-Boron.
3) Correction of atomic mass:-
?Atomic mass of beryllium was corrected from 13.5 to 9. Atomic mass of indium, gold and platinum were also corrected.
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2. He left gaps for the elements yet to be discovered.
3. He predicted the properties of those yet to be discovered elements and thus, helped in the discovery of other elements later on.
4. He named these elements by prefixing a sanskrit numeral 'Eka' (one), 'Divi' (two), 'Tri' (three), etc.to the names of the preceding preceding similar element in the same group. For example : eka - boron, eka - aluminium, eka - silicon, divi - manganese, eka - tantulum, etc.
All these elements were discovered later and did have the properties similar to those predicted by mendeleev. For example -
PROPERTIES EKA - ALUMINIUM GALLIUM
Atomic mass 68 69.7
Formula of oxide E2O3 Ga2O3
Formula of chloride ECL3 GaCl3
Demerits -
1. Although most of the elements were placed in the order of increasing atomic weights but this increasing order could not be maintained in all cases. For example - Cobalt atomic mass is 58.93u preceded Nickel 58.71u preceded Tellurium 127.6 preceded Iodine 126.90, thus he couldnt maintain similarities in properties.
2. Mendeleev periodic table did not provide space for noble gases which were discovered later.
3. Moreover there was no space left for isotopes in medeleev's periodic table.
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