The electrons in the atoms of four elements A, B, C and D are distributed in three shells having, 3, 5 and 7 electrons in the outermost shell respectively

The electrons in the atoms of four elements A, B, C and D are distributed in three shells having, 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.

Elements Valence electrons Period
A 1 3
B 3 3
C 5 3
D 7 3

Electronic configuration of A: Is2 2s2 2p6 3s1
Electronic configuration of D: Is2 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

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