The periodic table organizes all known chemical elements, arranging them to highlight recurring patterns in their properties. This structure consists of rows and columns. The horizontal rows that span the table are formally known as Periods.
There are seven distinct periods, numbered one through seven. Elements within a period are arranged in order of increasing atomic number from left to right, reflecting the underlying electron structure of the atoms.
Defining Periods
The Period number of an element directly corresponds to the number of electron shells, or principal energy levels, its atoms possess. For instance, any element in Period 3 has three main energy shells surrounding its nucleus where electrons reside.
As one moves across a Period, the number of protons in the nucleus and the number of electrons orbiting the nucleus both increase by one for each successive element. Crucially, these additional electrons are being added to the same outermost electron shell, not a new one. This consistent number of electron shells is the fundamental chemical meaning conveyed by the Period designation.
Distinguishing Periods from Groups
While the horizontal rows are Periods, the vertical columns are called Groups, or sometimes Families. Elements belonging to the same Group share a defining characteristic: they have the same number of valence electrons in the outermost shell.
This shared count of valence electrons is why elements within the same Group exhibit similar chemical properties and reactivity. In contrast, elements within the same Period have different numbers of valence electrons, causing them to display a wide range of chemical behaviors, from highly reactive metals on the left to inert gases on the far right. The Period defines the number of electron shells, while the Group defines the number of valence electrons.
Key Properties of Elements within a Period
Moving from left to right across a Period reveals systematic changes in the properties of the elements, known as periodic trends. One of the most noticeable trends is the steady decrease in atomic radius. Although each successive element gains an electron, the simultaneous increase in the number of protons in the nucleus results in a greater positive charge.
This stronger positive nuclear charge pulls the electron cloud inward, making the atom physically smaller. Because the electrons are being added to the same main energy shell, the shielding effect of inner electrons remains relatively constant, allowing the increasing nuclear charge to have a greater effect on the outermost electrons.
Another trend is the increase in ionization energy, which is the energy required to remove an electron from an atom. As the atomic radius shrinks across the Period, the outermost electrons are held more tightly by the increasingly positive nucleus. Consequently, more energy is needed to overcome this stronger attraction and pull an electron away from the atom.
Finally, electronegativity also increases across a Period. Electronegativity is an atom’s tendency to attract a shared pair of electrons toward itself when forming a chemical bond. The smaller atomic size and greater effective nuclear charge make it easier for atoms on the right side of the table to attract and hold electrons, resulting in higher electronegativity values.