Why did Mendeleev have gaps in his periodic table?

Question

(i) Why did Mendeleev have gaps in his periodic table? 

(ii) How does electronic configurations of atoms change in a period with increase in atomic number?

Answer 1

(i) Mendeleev left some gaps in his periodic table for the elements that were not known at his time. These gaps were left for the elements which could match with other elements of that group but had not discovered by that time.

  (ii) On moving from left to right along a period one electron each is added successively to the same valence shell of atom. From left to right, valence electrons increase from 1 to 8 (1 to 2 in first period).