The periodic table is one of the most recognized organizational tools in science, providing a systematic arrangement for all known chemical elements. This chart groups elements based on shared characteristics and increasing atomic number, allowing scientists to predict properties and chemical behavior. The horizontal rows are formally known as Periods, a name describing the repeating pattern of element properties as you move across the table.
The Naming and Layout of Periodic Rows
The periodic table is structured with seven main horizontal rows, numbered 1 at the top to 7 at the bottom. Each element’s position is determined by its atomic number, which steadily increases as you move from left to right along any row. The term “periodic” references the cyclical, or repeating, trends in chemical properties that occur as a new row begins.
While the main body contains seven periods, two separate rows—the Lanthanides and Actinides—are often placed below the main structure. These elements are not distinct periods but are actually part of the sixth and seventh periods, respectively. They are detached from the main table simply for formatting convenience to prevent the chart from becoming too wide. The Lanthanides belong within Period 6, following Barium, and the Actinides belong within Period 7, following Radium.
What Periodic Rows Represent
The period number an element occupies has a direct physical meaning related to the structure of its atoms. Specifically, the number indicates the highest energy level, or electron shell, occupied by the element’s electrons. For instance, any element in Period 3, such as Sodium (Na) or Argon (Ar), possesses three occupied electron shells surrounding its nucleus.
Electron shells can be thought of as specific orbits or layers where electrons reside. Atoms in Period 1, like Hydrogen and Helium, have only a single electron shell, while elements in Period 7 have seven occupied electron shells. Moving down the table, each subsequent period adds another electron shell, causing the atoms to become physically larger.
Moving across a single period from left to right, elements maintain the same number of electron shells. However, the number of protons in the nucleus increases by one for each step. This gradual increase in positive charge pulls the electron shells closer to the center, causing the atomic size to decrease slightly across the period. This systematic change in structure drives the predictable shifts in chemical and physical properties, known as periodicity.
How Periods Differ from Groups
Understanding the role of periods requires contrasting them with the table’s other primary organizational feature: the vertical columns, called Groups. Periods run horizontally, while groups run vertically, representing a different aspect of an element’s chemistry. There are 18 groups, and elements within the same group share similar chemical behaviors.
The key distinction lies in what each feature represents about the atom’s electron configuration. The period number tells you the total number of electron shells an atom has. In contrast, the group number indicates the number of valence electrons, which are the electrons found in the outermost shell. Therefore, elements in the same group share the same number of reactive outer-shell electrons, leading to similar chemical properties.