(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) Gaps were left for undiscovered elements in the periodic table.
(b) (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.
( c) Number of shells remains the same, number of valence electrons goes on increasing from left to right in a period till octet is complete. e.g.,