What do horizontal rows in the periodic table represent?

Horizontal rows in the periodic table are known as periods. Each period corresponds to the principal energy level of the electrons in the atoms of the elements listed in that row. As you move across a period from left to right, the atomic number increases, which reflects the addition of protons in the nucleus and, typically, the addition of electrons in the outer energy levels.

Elements within the same period display a gradual change in properties, such as electronegativity and atomic radius, which are influenced by their increasing nuclear charge. For example, in the second period, elements transition from lithium, a metal, to neon, a noble gas, showcasing a variety of properties as you progress through different types of elements.

In contrast, groups, which are the vertical columns, categorize elements that share similar properties and have the same number of electrons in their outer shell. Understanding that horizontal rows indicate periods helps in grasping the overall structure and organization of the periodic table, thus reinforcing the knowledge of elemental properties and trends.