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.

Answer 1

The elements A, B, C, D belong 3rd period.
EC of A is 2,8,1
EC of D is 2,8,7
When A and D combine AD is the molecular formula.

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Answer 2

The period of element with outermost electrons:

$1=1^{\text { st }} \text { period }=\mathrm{A}$

$3=2^{\text { nd }} \text { period }=\mathrm{B}$

$5=2^{\text { nd }} \text { period }=\mathrm{C}$

$7=2^{\text { nd }} \text { period }=\mathrm{D}$

Electronic configuration of:

$\mathrm{A}=1 \mathrm{s}^{1}$

$\mathrm{D}=1 \mathrm{s}^{2}, 2 \mathrm{s}^{2}, 2 \mathrm{p}^{5}$

The molecular formula of the compound formed when A and D combine will be $\mathrm{DA}_{3}$ as A has one valence electron and D has 5 valence electrons hence to get the octet complete it has to make $\mathrm{D} \mathrm{A}_{3}$.