(i) Mendeleev arranged 63 elements known at that time in a periodic table which was known as the Mendeleev's periodic table. The table consisted of eight vertical columns called ‘groups’ and horizontal rows called ‘periods'.
LIMITATIONS OF MENDELEEV'S PERIODIC TABLE:
• He could not assign a correct position of hydrogen in his periodic table, as the properties of hydrogen resemble both with alkali metals as well as with halogens.
• The atomic masses of the elements in the periodic table did not increase regularly in going from one element to the next.
So, it was not possible to predict how many elements could be discovered between the two elements.
These shortcomings were removed in the modern periodic table as:
• The position of hydrogen-
Hydrogen is placed in the centre of the periodic table on the upper side, in between the alkali metals and halogens. Because of this, it resembled the properties of both the groups.
• Problem with atomic masses-
The elements in the modern periodic table were arranged on the basis of their atomic numbers and not atomic masses.
(ii) ELEMENT X:
Atomic number =12
Electronic configuration of X: 2,8,2
Atomic number =16
Electronic configuration of Y: 2,8,6
They would form an electrovalent (ionic) bond bearing the formula XY.
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