Read First –
(1) Systematic Study of the elements
Mendeleev’s periodic table was developed in accordance to-
(a) increasing atomic masses.
(b) grouping of similar elements together.
By the way, the study of elements became quite simpler, as if properties of one element in a particular group were known, properties of others could easily be assumed. Thus, there was a regular gradation in the properties of elements in groups and periods.
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MATTER / ATOMS AND MOLECULES / STRUCTURE OF THE ATOM / CHEMICAL REACTIONS AND EQUATIONS / ACIDS, BASES AND SALTS / METALS AND NON-METALS / CARBON AND ITS COMPOUNDS / PERIODIC CLASSIFICATION OF ELEMENTS
(2) Prediction of new elements and their properties
In Mendeleev’s periodic table, some vacant spaces were left with strong prediction for their discovery later on. For e.g.,
The predicted properties including atomic masses of the then discovered elements were found almost similar to the actual properties.
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(3) Accommodation of Nobel gases
Nobel gases were discovered later and were added in a new group called zero group which was not present in original Mendeleev’s periodic table. So, that original periodic table was not disturbed.
For e.g. Co (at. Mass = 58.9 higher) was placed before Ni (at. Mass = 58.7 lower) so that Co could be in the same column as Rhodium which closely resembles it in properties.
(4) Correction of doubtful atomic masses
Mendeleev’s periodic table helped in correction of atomic weights of certain elements such as Beryllium (Be, group IIA) , gold (Au, group IB) and platinum (Pt, group VIII), based on their position in periodic table.
(1) Anamolous position of Hydrogen
(2) Position of Isotopes could not be explained
Isotopes were discovered later. They were atoms of same elements having similar chemical properties, but different atomic masses.
3) Atomic masses do not increase in a regular manner of two successive elements.
So, it was not possible to predict the number of missing elements between two known elements, especially when we consider the heavier elements.
(4) Anomalous position of some pairs of elements
In some cases, Mendeleev placed elements according to their similarities in properties and not in increasing order of their atomic masses . Some elements having higher atomic masses had been placed before an element having lower atomic masses.
- Co(58.9) and Ni(58.7), Co was placed before Ni.
- Te (127.6) and I(126.9) Te was placed before I.
Thus, the position of these elements was not justified.
(5) Some similar elements were separated and dissimilar elements were grouped together.
Cobalt and Mercury; Gold and Platinum; Lead and Barium; Silver and thallium; Manganese and halogens were placed separately although similar in properties. Co was placed in group I although it did not resemble the elements in the group.
Thus, Mendeleev’s Periodic Table provided the basis of modern periodic classification.
Modern Periodic Table