Question

An atom in the ground state has seven valence electrons. This atom could be an atom of which element?
(1) calcium (3) oxygen
(2) fluorine (4) sodium

Answer 1

If you look up the valence electron table, only fluorine has seven valence electrons.
Hope this would help~

Answer 2

The valence electrons of an element is usually determined by its atomic number. The valence elctrons are the electrons that have the highest energy level. 

Fluorine has 7 valence electrons in the ground state.