Answer:
D. Na has one fewer valence electron than Ca.
Explanation:
Elements in different groups has been grouper based on the number of valence electrons they possess. Valence electrons refers to the electrons at the outermost part of the shell of an atomic element.
Sodium (Na) belongs to group 1 on the periodic table meaning that it has one number of valence electrons while Calcium (Ca) belongs to group 2 meaning that it has two number of valence electrons. Hence, Na has one fewer valence electron than Ca.
