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Question
The elements A, B, C, D and E have atomic numbers 9, 11, 17, 12 and 13 respectively. The pair of elements which belongs to the same group of the periodic table is:
(a) A and B
(b) B and D
(c) A and C
(d) D and E
Solution
(c) A and C
Electronic configuration of A (9) : K L
2 7
Electronic configuration of C (17): K L M
2 8 7
Group number of A and B = valence electron + 10
= 7+10 = 17.
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