Since the electrons in the atoms of four elements A, B, C and D are distributed in three shells which are having 1, 3, 5 and 7 electrons in the outermost shell. So, the electronic configuration of elements A, B, C and D is as follows:
A: 2, 8, 1
B: 2, 8, 3
C: 2, 8, 5
D: 2, 8, 7
Since all the elements have three sub-shells, so they belong to 3rd period.
We need to find the valency of element A and D, to find the molecular formula of the compound
Since, element A has 1 valence electron, therefore valency of A is 1. Also, Element D has 7 valence electrons, so valency of D is 1.
Cross multiplying the valences of element A and element D:
Therefore, the compound formed is: AD
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