Question

Q. No.-30) Write any three limitations for the following Periodic
Table:-
a) Dobereiner's Triads.
b) Newland's Law of Octaves.
c) Mendeleeve's Periodic Table.
Also define the "Modern Periodic Law".​

Answer 1

Explanation:

a)

it failed to arrange all the then known elements in the form of triads of elements having similar chemical properties. Dobereiner could identify only three triads from the elements known that time.

b)

Newlands law of octaves was applicable to the classification of elements upto calcium only. After calcium every eighth element did not possess the properties similar to that of the first element.

(ii) Newlands assumed that only 56 elements existed in nature and no more elements would be discovered in the future. But later on, several new elements were discovered whose properties did not fit into Newlands’ law of Octaves.

(iii) In order to fit elements into his table, Newlands put even two elements together in one slot and that too in the column of unlike elements having very different properties.

For example, the two elements cobalt (Co) and nickel (Ni) were put together in just one slot and that too in the column of elements like fluorine, chlorine and bromine which have very different properties from these elements.

(iv) Iron (Fe) element which resemble elements like cobalt and nickel in properties, was placed far away from these elements

c)

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.

Morden preodic law

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