Final answer:
The horizontal rows on the periodic table are called periods, and they are numbered from 1 to 7. Each period represents a range of elements with a specific number of electrons in their sublevels.
Step-by-step explanation:
The horizontal rows on the periodic table of elements are called periods. Each period is numbered from 1 to 7, with the 1st period containing only two elements: hydrogen and helium. The length of each period depends on the number of electrons needed to occupy the sublevels that fill the period.