Dobereiner's Triads, Newlands Octaves and Mendeleev's Periodic Table Class 11 Notes | EduRev

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Development of modern periodic table

Certain groups of elements have similar chemical properties; we classify elements as members of the same chemical family. Such similarity in the properties of elements has led scientists to classify them in such a form where one can study them in an organized manner. Such an arrangement is called a Periodic Table. Some of the significant earlier attempts are discussed below.

Dobereiner's Triads:

He arranged similar elements in the groups of three elements called triads, in which the atomic mass of the central element was merely the arithmetic mean of atomic masses of the other two elements or all the three elements possessed nearly the same atomic masses

Dobereiner`s Triads, Newlands Octaves and Mendeleev`s Periodic Table Class 11 Notes | EduRev

It was restricted to few elements, therefore, discarded

Newland's Law of Octaves:

He was the first to correlate the chemical properties of the elements with their atomic masses.

According to him if the elements are arranged in the order of their increasing atomic masses the eighth element starting from the given one is similar in properties to the first one.

This arrangement of elements is called as Newland's law of Octave.

Dobereiner`s Triads, Newlands Octaves and Mendeleev`s Periodic Table Class 11 Notes | EduRev

This classification worked quite well for the lighter elements but it failed in the case of heavier elements and, therefore, discarded.

Lother Meyer's Classification

He determined the atomic volumes by dividing atomic mass with its density in solid states.

He plotted a graph between atomic masses against their respective atomic volumes for a number of elements. He found the following observations.

(i) Elements with similar properties occupied similar positions on the curve.

(ii) Alkali metals having larger atomic volumes occupied the crests.

(iii) Transitions elements occupied the troughs.

(iv) The halogens occupied the ascending portions of the curve before the inert gases.

(v) Alkaline earth metals occupied the positions at about the midpoints of the descending portions of the curve.

Dobereiner`s Triads, Newlands Octaves and Mendeleev`s Periodic Table Class 11 Notes | EduRev

Lothar Meyer's Arrangement of Elements

On the basis of these observations, he concluded that the atomic volumes (a physical property) of the elements are a periodic function of their atomic masses.

It was discarded as it lacks practical utility.

Mendeleev's Periodic Table:

Mendeleev's Periodic's Law

According to him the physical and chemical properties of the elements are a periodic function of their atomic masses. He arranged then known elements in order of their increasing atomic masses considering the fact that elements with similar properties should fall in the same vertical columns and leaving out blank spaces where necessary.


Merits of Mendeleev's Periodic table:

(i) It has simplified and systematized the study of elements and their compounds.

(ii) It has helped in predicting the discovery of new elements on the basis of the blank spaces given in its periodic table.

Mendeleev predicted the properties of those missing elements from the known properties of the other elements in the same group. Eka - Aluminium, and Eka-silicon names were given for gallium and germanium (not discovered at the time of Mendeleev). Properties predicted by Mendeleev for these elements and those found experimentally were almost similar.

Dobereiner`s Triads, Newlands Octaves and Mendeleev`s Periodic Table Class 11 Notes | EduRev


Property

eka-aluminium(predicted)

gallium 

(found)

eka-silicon 

(predicted)

germanium

(found)

Atomic Mass
68
70
72
72.6
Density / (g/cm3)
5.9
5.94
5.5
5.36
Melting Point (K)
Low
30.2
High
1231
Formula of Oxide
E2O3
Ga2O3
EO2
GeO2
Formula of Chloride
ECl3
GaCl3
ECl4
GeCl4

 

(iii) Atomic weights of elements were corrected. The atomic weight of Be was calculated to be 3 × 4.5 = 13.5 by considering its valency 3, was correctly calculated considering its valency 2 (2 × 4.5 = 9)


Demerits In Mendeleev's Periodic Table:

(i) Position of hydrogen is uncertain. It has been placed in group 1 and 17th group because of its resemblance with both the groups.

(ii) No separate positions were given to isotopes.

(iii) Anomalous position of lanthanides and actinides in periodic table.

(iv) Order of increasing atomic weights is not strictly followed in the arrangement of elements in the periodic table. For e.g.-Ar(At.wt.39.94) is placed before K(39.08) and Te (127.6) is placed before I (126.9)

(v) Similar elements were placed in different groups(CuIB and Hg IIB) and the elements with different properties were placed in the same groups(alkali metals IA and coinage metals IB)

(vi) It didn't explain the cause of periodicity.

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