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A) Mendeleev’s periodic table is, based on relation of properties of elements as dependent on the atomic weight of element. But Modern periodic table considers atomic number as the fundamental property that decides the properties of elements.
Modern periodic table does correct defects of the Mendeleev’s periodic table. For example, in the Mendeleev’s periodic table, in the element pairs, Argon-potassium, cobalt-nickel, tellurium-iodine and thorium and protactinium, elements with higher atomic mass precedes the element with lower atomic weight. Though it is the right places for them but is against the Mendeleev’s periodic law.
These elements atomic number shows the reverse order compared to atomic mass. The supposed to be wrong positions in Mendeleev’s table has the right explanation justifying their positions.
Uneven grouping of elements:
In Mendeleev’s periodic table, coinage metals of copper, silver, and gold are, grouped together with very active alkali metals. Manganese metal was, grouped with halogens in the seventh group. The defects are rectified in the Modern periodic table.
Position of isotopes:
Isotopes with higher atomic weights could not, be accommodated in the Mendeleev’s table. As isotopes have same atomic number with the stale atom they do not need any separate location in the modern periodic table.
No reasons were, offered for the triad elements of viii group. No such special grouping is given in the modern table.
Mendeleev’s periodic table was for the arrangement of sixty-three elements known at that time.
Modern table accommodates all the 118- natural and synthetic elements.
Atomic number is much fundamental property distinguishing each element and hence a better basic nature to decide the physical and chemical properties of elemental atoms than atomic weights.
B) When hydrogen loses an electron and forms a cation, it resembles alkali metals but when it gains an electron and becomes a uni-negative ion it shows similarity to halogens. Looking at these properties the position of hydrogen in the periodic table was a big question.
C) Mendeleev left gaps in his periodic table because the properties of known elements predicted other, as-yet-undiscovered, elements in these locations. As Mendeleev organized his periodic table, he recognized that these gaps would be filled as future scientists identified new elements
D) Limitations of the Mendeleev periodic table are listed below.
Elements with large differences in properties were included in the same group. for example, hard metals like copper and silver were included along with soft metals like sodium and potassium.
No proper position could be given to the element hydrogen. In the periodic table, the location of hydrogen is uncertain. It was put with alkali metals within 1A class, but certain hydrogen properties are close to those of halogens. So, it can also be put for halogens in the band.
The increasing order of atomic mass was not strictly followed throughout. For example in cobalt and nickel & Tellurium and Indium.
As isotopes are atoms of the same element having different atomic masses, they should have been given different positions while arranging them in the order of atomic mass. But this was not done.
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E) Variation Of Oxidation State Along a Period While moving left to right across a period, the number of valence electrons of elements increases and varies between 1 to 8. But the valency of elements, when combined with H or O first, increases from 1 to 4 and then it reduces to zero
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