What term is used to describe the horizontal rows on the periodic table?

The term used to describe the horizontal rows on the periodic table is "periods." Each period represents a different level of electron shell filling, which corresponds to the energy levels of the electrons in an atom. As you move from left to right across a period, the atomic number of the elements increases, indicating that each successive element has one more proton and one more electron than the previous element. This arrangement allows for the periodic trends in properties such as atomic radius, ionization energy, and electronegativity, which change in a predictable manner across periods.

This systematic organization is fundamental to understanding the behavior of elements and their chemical properties. In contrast, groups refer to the vertical columns of the periodic table, where elements share similar chemical properties due to their similar valence electron configurations.