What are the horizontal rows on the periodic table called?

The horizontal rows on the periodic table are called periods. Each period represents a new electron shell being filled as you move from left to right across the table. The number of the period corresponds to the highest energy level that is occupied by electrons in the elements within that row. For example, elements in the first period have their electrons in the first energy level, while those in the second period fill the second energy level, and so on. This systematic arrangement is essential for understanding the trends in the properties of elements, such as electronegativity and atomic size, which change as you move across a period.