What are the horizontal rows on the periodic table called?

The horizontal rows on the periodic table are referred to as periods. Each period corresponds to the principal energy levels of the elements, which indicate the highest electron shell that is occupied by electrons in an atom of the elements within that row. As you move from left to right across a period, the atomic number increases, and the properties of the elements change progressively, usually from metals to non-metals. This trend reflects the gradual filling of the outer shells of electrons, which in turn influences an element's chemical behavior. Understanding the arrangement of elements in periods is essential for grasping the relationships between different elements and predicting their chemical properties.