Periods are what on the periodic table?

• A. Vertical columns
• B. Sets of elements with similar properties
• C. Horizontal rows indicating the highest energy level the electrons occupy at ground state
• D. The total energy of the electrons in the atom

Answer: (C)

Periods are horizontal rows on the periodic table that indicate the principal energy level (the highest energy level) that the electrons occupy in the ground state. Elements in the same period share the same outer-shell energy level, so the period number corresponds to that shell (for example, n = 2 for the second period, n = 3 for the third, and so on). As you move across a period, you add electrons within that same shell until the next shell begins in the following period. The other descriptions refer to groups (vertical columns) or to properties shared by elements, not to the row structure that shows the outer energy level.