The periodic table organizes all known chemical elements in order of increasing atomic number (the count of protons in an atom’s nucleus). Its structure consists of horizontal rows and vertical columns. A row in the periodic table is known as a period. There are seven periods, starting at the top with Period 1 and moving down to Period 7. The placement of an element within a period reveals crucial information about its atomic structure.
What Defines the Period Number
The number assigned to each period has a direct physical meaning tied to the atom’s electron structure. The period number tells you the number of electron shells that the elements in that row possess. For example, every element listed in Period 3, such as Sodium (Na) and Chlorine (Cl), has three main electron shells surrounding its nucleus.
These electron shells can be visualized like the layers of an onion, with each subsequent shell being farther away from the central nucleus. As you move from Period 1 down through Period 7, elements gain one additional layer of electrons. This steady increase defines the vertical progression of the periodic table. The addition of a new shell for each period is why atoms generally become physically larger as you move toward the bottom of the table.
Distinguishing Periods from Groups
While periods organize the elements horizontally, the other major organizing principle involves the vertical columns, which are known as groups. Elements within a group share similar chemical behaviors because they all have the same number of valence electrons, which are the electrons in the outermost shell.
In contrast, elements across a period do not share the same number of valence electrons, but rather the same number of electron shells. For example, in Period 2, Lithium (Li) has one valence electron, while Neon (Ne) has eight. The consistent number of electron shells in a period contrasts with the consistent number of valence electrons found in a group, which is the key difference in the table’s organization.
Changes in Element Properties Across a Row
Moving from left to right across any given period reveals predictable and systematic changes in element properties, a phenomenon known as the periodic law. These changes are driven by the fact that while the number of electron shells remains constant, the number of protons in the nucleus increases by one with each step. This increasing positive charge pulls the electron cloud inward more strongly, which affects the behavior of the atom.
One of the most noticeable trends is the decrease in atomic radius as you move across a period. Since new electrons are added to the same main shell, the increasing positive nuclear charge exerts a greater attractive force on all the electrons. This pulls the electron cloud closer to the nucleus, resulting in the atom becoming progressively smaller from left to right.
The strength of this nuclear pull also influences an atom’s tendency to attract or lose electrons, which is described by two related properties. Ionization energy, which is the energy required to remove an electron from an atom, generally increases across a period. Since the electrons are held more tightly by the increasing nuclear charge, it requires more energy to pull the outermost electron away from the atom.
Similarly, electronegativity, the measure of an atom’s ability to attract electrons in a chemical bond, also increases across a period. Elements on the left, such as the alkali metals, have low electronegativity and easily give up their single valence electron. Elements on the far right, like the halogens, have high electronegativity because their outer shell is nearly full, making them strongly inclined to attract and gain an electron.
The change in these properties also dictates the shift in metallic character across a period. Elements on the far left, such as the alkali and alkaline earth metals, are highly metallic, characterized by their readiness to lose electrons and conduct electricity. As you move to the right, this metallic nature steadily decreases, transitioning through metalloids and culminating in the nonmetals, which prefer to gain electrons. This predictable, continuous shift is the defining feature of traversing a period.