What is the horizontal grouping of elements in the periodic table called?

The horizontal grouping of elements in the periodic table is referred to as a "period." In the context of the periodic table, periods are the rows that indicate the number of electron shells in the atoms of the elements found within that row. As you move across a period from left to right, the atomic number increases, which corresponds to an increase in the number of protons and electrons. The properties of the elements also change systematically across the periods, reflecting the periodic law.

This fundamental organization helps chemists understand and predict the behavior of elements as they relate to one another, underscoring the importance of periods within the framework of the periodic table.