Verified Answer :

The atom that has 7 valence electrons is A. fluorine.
It is in the halogens group. 
It needs 1 more electron to become stable, therefore, it is highly reactive.

Answer: Option (A) is the correct answer.

Explanation:

An atom who has seven valence electrons in its ground state will have the following electronic configuration in its sub shells as 2, 7.

Thus, there will be in total 9 electrons and it is known that fluorine has atomic number 9.

Whereas sodium has 2, 8, 1 electrons in its sub-shell.

Calcium has 2, 8, 8, 2 electrons in its sub-shell and oxygen has 2, 6 electrons in its sub-shell.

Thus, we can conclude that out of the given options, fluorine is the element whose atom in the ground state has seven valence electrons.

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