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Question

# The atomic numbers of three elements A, B and C are given below : Element Atomic number A 5 B 7 C 10 (i) Which element belongs to group 18? (ii) Which element belongs to group 15? (iii) Which element belongs to group 13? (iv) To which, period/periods do these elements belong?

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Solution

## (i) Element C belongs to group 18. Electronic configuration of element C (10) = 2,8 Therefore, group number = valence shell + 10 = 8+10 = 18. (ii) Element B belongs to group 15. Electronic configuration of element B (15) = 2,8,5 Therefore, group number = valence shell + 10 = 5+10 = 15. (iii) Element A belongs to group 13. Electronic configuration of element A (5) = 2,3 Therefore, group number = valence shell + 10 = 3+10 = 13. (iv) The elements A, B and C all belong to period 2 because all three have the same valence shell, i.e, L.

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