The number of electron shells in an atom is determined by its what?

• A. Group
• B. Period
• C. Block
• D. Periodic Table

Answer: (B)

Electrons fill energy levels called shells, and the number of shells an atom has corresponds to the period it sits in on the periodic table. That row tells you how many distinct energy levels are occupied in a neutral atom, so the period determines the total number of electron shells.

For example, atoms in the first period have one shell, those in the second period have two shells, and so on. The other aspects—group relates to the number of valence electrons, block to which subshell is being filled, and the periodic table as a whole to the organization of elements—don’t directly indicate how many shells an atom contains.