(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. ORThe electrons in the atoms of four elements $A, B, C$ 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 $B$ and $D$, and the molecular formula of the compound formed when $B$ and $D$ combine.