The group number represents what property?

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

Answer: (A)

The group number matches the number of electrons in the outer shell for the main-group elements. Those outer-shell electrons, called valence electrons, determine how atoms bond and behave in reactions. Elements in the same group have the same valence electron count, which is why they share similar chemical properties. For example, a group with 1 valence electron tends to lose it and form a +1 ion, while another group with 7 valence electrons tends to gain one to complete a stable shell. This idea isn’t about how many electron shells the atom has (that’s the period), nor about atomic mass or the number of protons (that’s the atomic number).