Given below are some elements of the modern periodic table. Atomic numbers of the elements are given in the parentheses: $A(4), B(9), C(14), D(19), E(20)$.
$(a)$ Select the element that has one electron in the outermost shell. Also,write the electronic configuration of this element.
$(b)$ Which two elements amongst these belong to the same group? Give reason for your answer.
$(c)$ Which two elements amongst these belong to the same period? Which one of the two has a bigger atomic radius?

(D) The element with one electron in the outermost shell is $D$ (atomic number $19$). Its electronic configuration is $2, 8, 8, 1$.
$(b)$ Elements $A$ (atomic number $4$,configuration $2, 2$) and $E$ (atomic number $20$,configuration $2, 8, 8, 2$) belong to the same group because both have $2$ valence electrons in their outermost shell.
$(c)$ Elements $D$ (atomic number $19$,period $4$) and $E$ (atomic number $20$,period $4$) belong to the same period. Between $D$ and $E$,$D$ has a bigger atomic radius because atomic radius decreases across a period from left to right due to an increase in nuclear charge.