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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?
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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