The electrons in the atoms of four elements $A, B, C$ and $D$ are distributed in three shells having $1, 3, 5$ and $7$ electrons in the outermost shell respectively. State the period in which these elements can be placed in the modern periodic table. Write the electronic configuration of the atoms of $A$ and $D$ and the molecular formula of the compound formed when $A$ and $D$ combine.

(A) $(i)$ Since all four elements have electrons distributed in three shells,they all belong to the $3^{rd}$ period of the modern periodic table.
$(ii)$ The electronic configuration is determined by filling the shells $(K=2, L=8, M=n)$:
For element $A$ (outermost shell has $1$ electron): $2, 8, 1$.
For element $D$ (outermost shell has $7$ electrons): $2, 8, 7$.
$(iii)$ Element $A$ has $1$ valence electron,so it forms a cation $A^+$. Element $D$ has $7$ valence electrons,so it gains $1$ electron to form an anion $D^-$. The compound formed by the combination of $A$ and $D$ is $AD$.