The horizontal rows of the periodic table are called periods.
Each period represents elements whose atoms have the same number of occupied electron shells, or principal energy levels. As you move from left to right across a period, the atomic number increases by one and the elements generally become less metallic. Their properties change in a repeating pattern, which is the basis of the periodic law.
The periodic table currently contains seven complete periods and 118 confirmed elements. The first period contains only hydrogen and helium. Periods 2 and 3 each contain eight elements, while periods 4 and 5 contain 18. Periods 6 and 7 are longer because they include the lanthanides and actinides, often displayed in two detached rows beneath the main table.
A common mix-up is to reverse periods and groups. Periods run horizontally; groups are the vertical columns. Elements in the same group often share similar chemical behavior because they have related outer-electron arrangements.