Question

Explain the merits and demerits of the following:-

Dobereiner's law of traids

Newland's law of octaves

Mendeleev's periodic table

Modern periodic table.

Answer 1

Newlands Law of Octaves has many limitations, which are discussed below.

This law was not applicable throughout the arrangement. It was applicable only till calcium.

Newlands assumed that only 56 elements would exist in nature and believed that no more elements would be discovered. However, several elements were discovered in the following years. These elements did not follow the Law of Octaves.

The positions of cobalt and nickel could not be explained according to Newlands Law of Octaves. He kept cobalt and nickel in the same slot. They were also placed in the same column as fluorine and chlorine, which have completely different properties.

The properties of iron are similar to those of cobalt and nickel. However, iron was placed away from them in a different column.

 

Merits of Newlands Law of Octaves____

Newlands law of octave provided a basis for the classification of element on the basis of having similar properties into groups of elements.

The arrangement of elements in tabular form was provided by Newlands law of Octaves.

Newlands law of octave was the first logically basis on the atomic weight.

Demerits of dobereiner’s triads system:

- This system fails because it does not hold good for all the elements discovered till date time. It is limited to only few elements.

- It does not completely explain the complete properties of elements like valency, oxidation state of elements which have variable oxidation state like phosphorus showing trivalent and pentavalent nature.

- It does not fit to transition elements.

The only advantage of Dobereiner’s research was that it made chemists look at elements in terms of groups of elements with similar chemical and physical properties. This eventually led to rigorous classification of elements and the modern periodic table of elements, as we now know it, was discovered.


Mendeleev's :


MERITS ARE

1. The classification of elements is based on the atomic number, which is a more fundamental property.

2. The reason for placing isotopes at one place is justified as the classification is on the basis of atomic number.

3. It explains the periodicity of the properties of the elements and relates them to their electronic configurations.

5. The position of the elements that were misfits on the basis of mass number (anomalous pairs like argon and potassium) could be justified on the basis of atomic number.

6. The lanthanides and actinides are placed separately at the bottom of the

periodic table.

7. The table is simple, systematic and easy way for remembering the properties of various elements as it is based on the electronic configuration.

Modern periodic table :

Modern periodic table has many benefits as it classifies the elements on the basis of electronic configuration of elements.

 By arranging the elements in this way it is easy to remember the elements in sequence. The elements are distributed in different blocks to make the study   easier.

 The elements present in the same group show similar physical and chemical properties as they have same valence electron in their last shell.

 Correct position to inert gases is given in Modern periodic table.

 It is easier to understand the properties of the elements as they are grouped into different blocks i.e. s, p, d and f.

 Accurate position to every element is provided in Modern periodic table.

 Metals, Non-metals and metalloids are grouped differently to make the study easier

Answer 2

Dobereiner's law of triads: According to which in certain triads the atomic mass of the central element was merely the arithmetic mean of the atomic masses of the other two elements and the properties of the middle element were in between those of the end members. These families of three elements became known as Dobereiner's Triads.

Merits-  

• From this classification, one point became clear that properties of the elements have some relationship with their atomic masses.

Demerits-

• In some triads, all the three elements possessed nearly the same atomic masses, hence the law was rejected.
• The concept was not applicable to all the known elements but only to a limited number of elements.

Newland's law of octaves: According to this law, the elements when arranged in order of their increasing atomic masses, the eighth suceeding element was the repetition of the first one like 8th note of the musical scale,i.e., after a difference of seven, elements with similar characteristics were observed.

Merits -

• The system worked quite well for the lighter elements. For example, H, F, Cl show similar properties and similarly Li, Na and K exhibit same characteristics.

Demerits -

• The system fails in the case of heavier elements.
• The properties of the elements were not taken into account and only the elements were arranged in order of ascending atomic masses.
• No space was left for the elements which were unknown at that time.
• Many elements were provided wrong positions on account of wrong values of atomic masses.

Mendeleev's periodic table: This law stated that ' If the elements are arranged in order of increasing atomic masses, their properties vary in definite manner from member to member of the series, but return more or less nearly to same value at certain fixed points in the series'.

Merits -

• Mendeleev presented the periodic table, only 63 elements were known and left 29 places for unknown elements.

Demerits -

• This table consists a number of defects such as anomalous pairs, position of isotopes, position of lanthanides and actinides, etc.

Modern periodic table: This law state that ' The physical and chemical properties of the elements are periodic function of their atomic numbers'.

Merits:

• The periodic table has reduced the study of 117 known elements. If we know the properties of a single member of a group, we can have good approximation of the properties of other members of this group.
• He predicted the property of new element.

Demerits-

• Couldn't explain the position of hydrogen.
• Elements showing marked dissimilarities are placed in one group.
• No attempt has been made to place metals and non metals seperately in the periodic table.