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.



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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all the elements belong to 3 period

Electeonic confoguration of
A= 2,8,1

A is magnesium
B is chlorine

so when they combine
it forms magnesium chloride.(AD)