What term describes horizontal rows in the periodic table?

The term that describes horizontal rows in the periodic table is "periods." Each period corresponds to the filling of a different electron shell around the nucleus of the atoms. As you move from left to right across a period, the atomic number increases, with each subsequent element having an additional proton and electron. This progression helps to visualize the periodic trends, such as electronegativity and atomic radius, which change across periods.

The other terms listed refer to different organizational structures in the periodic table. Groups refer to the vertical columns, where elements share similar chemical properties due to their similar valence electron configurations. Families typically imply groups with shared characteristics, often encompassing elements in the same vertical column. Clusters is not a standard term used in the context of the periodic table. Therefore, "periods" is the most accurate answer when referring to the horizontal arrangement of elements.