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Question
The electrons in the atoms of four elements A, B, C and D are distributed in three shells having 1, 3, 5 and 7 electrons in the outermost shell respectively. State the period in which these elements can be placed in the modern periodic table. Write the electronic configuration of the atoms of A and D and the molecular formula of the compound formed when A and D combine.
Solution
| Elements | Valence electrons | Period |
| A | 1 | 3 |
| B | 3 | 3 |
| C | 5 | 3 |
| D | 7 | 3 |
Electronic configuration of A: 1s2 2s2 2p6 3s1
Electronic configuration of D: 1s2 2s2 2p6 3s2 3p5
Molecular formula of the compound formed with A and D:
Atomic number of A = 11
Electronic configuration A = 2,8,1
Number of valence electrons of A = 1
Valency of A = 1
Atomic number of D = 17
Electronic configuration D = 2,8,7
Number of valence electrons of D = 7
Valency of D = 8-7 = 1
So, the formula of the compound formed when elements A and D combine is AD.
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