(a) List any three observations which posed a challenge to Mendeleev's Periodic law.
(b) How does the metallic character of elements vary on moving from
(i) left to right in a period,
(ii) from top to bottom in a group of the Modern Periodic Table?
Give reason for your answer.
OR
The electrons in the atoms of four elements $\mathit{A},\mathit{B},\mathit{C}$ and D are distributed in three shells having 1, 3, 5 and 7 , electrons respectively in their outermost shells. Write the group numbers in which these elements are placed in the Modern Periodic Table. Write the electronic configuration of the atoms of $\mathit{B}$ and $\mathit{D}$, and the molecular formula of the compound formed when $\mathit{B}$ and $\mathit{D}$ combine.