What is the term for each row in the periodic table?

The term for each row in the periodic table is "period." In the context of the periodic table, a period refers to a horizontal row of elements, and elements within the same period share the same highest energy level or shell of electrons. For instance, the first period includes hydrogen and helium, both of which have their outermost electrons in the first energy level.

This organization is important because it reflects periodic trends in properties such as atomic size, ionization energy, and electronegativity, which often change in a predictable manner as you move across a period from left to right. In contrast, a "group" refers to the vertical columns of the periodic table, where elements have similar chemical properties due to having the same number of valence electrons. Thus, understanding the distinction between periods and groups helps in grasping the overall structure and trends within the periodic table.