The modern long-form periodic table has seven periods.
A period is a horizontal row, and the period number corresponds broadly to the highest principal electron shell occupied by an element’s electrons in its ground state. Period 1 contains only hydrogen and helium, while later periods contain progressively more elements.
Periods 6 and 7 are especially long because they include the lanthanides and actinides. These two series are commonly printed in separate rows beneath the main body to keep the table compact, but they remain part of periods 6 and 7. Period 7 ends with oganesson, element 118.
Periods are different from groups, which are vertical columns. The table has 18 groups but only seven periods. This distinction is a frequent source of confusion: group membership emphasizes similar outer-electron patterns, while a period tracks a principal electron-shell level.