(a) Why did Mendeleev have gaps in his periodic table ?
(b) State any three limitations of Mendeleev's classification ?
(c) How does electronic configurations of atoms change in a period with increase in atomic number?

Explanation

a) Mendeleev.s periodic law states that "the physical and chemical properties of all elements are the periodic function of their atomic masses"".
b) Gaps were left for undiscovered element in the Mendeleev.s periodic table.
c).i) Position of hydrogen was not justified ii) Increasing order of atomic mass could not be maintained iii) Isotopes have similar chemical properties but different atomic masses, they cannot be given separate places.
d) Number of shells remain the same, number of valence electrons goes on increasing from left to right in a period till octet is complete.