An atom's number of valence electrons also reflects its which property?

• A. Period
• B. Group
• C. Mass number
• D. Atomic number

Answer: (B)

Valence electrons determine how an atom tends to bond and react chemically. For the main part of the periodic table, the number of valence electrons matches the group (the column) the element sits in. That’s why the same group elements share similar chemistry: they have the same outer-electron count.

Period tells you how many electron shells an atom has, mass number is the total count of protons and neutrons, and atomic number is the number of protons (the element’s identity). So the number of valence electrons best reflects the group.