Concept:The group number of an element in the periodic table directly shows the number of valence electrons in its outermost shell.
Explanation:Valence electrons are the electrons in the outermost energy level of an atom.
These electrons are responsible for the chemical properties of the element.
In the periodic table, elements are arranged in vertical columns called groups.
Elements belonging to the same group have the same number of valence electrons.
For main-group elements, the group number equals the number of valence electrons.
For example, group
1 elements have
1 valence electron, while group
17 elements have
7 valence electrons.
The period number, on the other hand, indicates the number of electron shells, not valence electrons.
Atomic mass is unrelated to electron arrangement.
Answer:A. group number