The number of electron shells is indicated by which periodic number?

• A. The period number
• B. The group number
• C. The atomic number
• D. The mass number

Answer: (A)

The period number tells you how many electron shells an atom has. Each new period on the periodic table corresponds to a new shell being added as you move down/through the table, so elements in a given period all share the same outer shell count. For example, elements in the second period have two electron shells, those in the third period have three shells, and so on. The group number relates to valence electrons, the atomic number to total protons (and electrons in a neutral atom), and the mass number to protons plus neutrons, none of which directly indicate the number of shells.