What do horizontal rows in the periodic table refer to?

Horizontal rows in the periodic table are known as periods or series. Each period represents a new level of electron shells surrounding the nucleus of the atoms represented in that row. As you move from left to right across a period, the atomic number increases, indicating that each successive element has one more proton and typically one more electron. This arrangement helps in understanding the periodic trends such as atomic size, ionization energy, and electronegativity, as elements within the same period show gradual changes in these properties.

In contrast, groups or families refer to the vertical columns in the periodic table, where elements share similar chemical properties due to having the same number of valence electrons. Blocks refer to the categorization of elements based on their electron configurations, such as s-block, p-block, d-block, and f-block. Therefore, the term that accurately describes horizontal rows is indeed periods or series.