Question

mendleev law and limitations and merits and demerits and modern periodic law

Answer 1

Mendeleev’s periodic law : This law states that the physical and chemical properties of the elements are the periodic function of their atomic masses. This means that when the elements are arranged in the order of their increasing atomic masses, the elements with similar properties recur at regular intervals. Such orderly recurring properties in a cyclic fashion are said to be occurring periodically. This is responsible for the name periodic law or periodic table.

Merits of Mendeleev’s periodic table : Mendeleev’s periodic table was one of the greatest achievements in the development of chemistry. Some of the important contributions of his periodic table are given below ;

(A) Systematic study of elements : He arranged known elements in order of their increasing atomic masses considering the fact that elements with similar properties should fall in the same vertical column.

(B) Correction of atomic masses : The mendeleev’s periodic table could predict errors in the atomic masses of certain elements were corrected. For example, atomic mass of beryllium was corrected from 13.5 to 9. Similarly, with the help of this table, atomic masses of indium, gold, platinum etc. were corrected.

(C) Mendeleev predicated the properties of those missing elements from the known properties of the other elements in the same group. Eka-boron, eka – aluminum and eka – silicon names were given for scandium, gallium and germanium (not discovered at the time of Mandeleev).

(D) Position of noble gases : Noble gases like helium (He), neon (Ne) and argon (Ar) were mentioned in many studies. However, these gases were discovered very late because they are very inert and are present in extremely low concentrations. One of the achievements of Mendeleev’s periodic table was that when these gases were discovered, they could be placed in a new group without disturbing the existing order.

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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Answer 2

The mendeleevs law of periodic law states that - The properties of the elements are the periodic function of their atomic masses.

The modern periodic law states that - the properties of the elements are the periodic function of their atomic numbers.

Limitations and demerits of mendeleevs periodic table

1.) There was no place for isotopes

2.) In many of the places, the elements were not placed in increasing atomic masses. for eg. Cobalt and nickel .

3.) there was no proper place for hydrogen

Merits of mendeleevs periodic table

1.) Mendeleev left spaces for elements which were not found a that time. He predicted that those elements will be found in subsequent years.

2.) This was the base for the formation of modern periodic table.

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