The elements A, B, C, D and E have atomic number 9, 11, 17, 12 and 13 respectively. Which pair of elements belong to the same group? [NCERT Exemplar] (a) A and B (b) B and D (c) A and C (d) D and E

(c)Electronic configuration of A (atomic number = 9) is 2, 7. Electronic configuration of B (atomic number = 11) is 2, 8,1. Electronic configuration of C (atomic number = 17) is 2, 8, 7. Electronic configuration of D (atomic number =12) is 2, 8, 2. Electronic configuration of E (atomic number =13) is 2, 8, 3. Elements which differ in atomic number by 8, i.e. 9 (A, fluorine) and 17 (C, chlorine) lie in the same group, i.e. group 17 (halogen).