The group number corresponds to the number of what in main-group elements?

• A. The number of outer shell electrons
• B. The number of electron shells
• C. The atomic mass
• D. The proton count

Answer: (A)

In main-group elements, the group number tells you how many electrons are in the outermost shell, the valence electrons. Those outer electrons are what drive bonding and chemical properties, so belonging to the same group means you have the same number of valence electrons. For example, group 1 elements have 1 valence electron, group 2 have 2, and groups 13–18 have 3–8 valence electrons respectively. The number of electron shells is related to the period, not the group; atomic mass varies with isotopes, and the proton count equals the atomic number, not the group. So the group number corresponds to the number of outer-shell electrons.