Limitation Of Mendeleev Classification

How did Mendeleev arrange the elements? He left empty places in his periodic table. Why? Explain the limitations of Mendeleev classification

(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?

a) Discuss any two achievements of the Modern Periodic Table. b) Discuss any two limitations of Mendeleev's classification. c) How were these removed in the Modern Periodic Table?