Final answer:
Each row of elements on the periodic table is called a period. Periods have different lengths, with the first period having 2 elements and later periods having 8 or 18 elements. Some periods are long and have a segment removed from beneath the main body of the table.
Step-by-step explanation:
Each row of elements on the periodic table is called a period. Periods have different lengths; the first period has only 2 elements (hydrogen and helium), while the second and third periods have 8 elements each. The fourth and fifth periods have 18 elements each, and later periods are so long that a segment from each is removed and placed beneath the main body of the table.