By David Chandler, Updated March 24, 2022
Jupiterimages/Photos.com/Getty Images
The periodic table arranges elements in order of increasing atomic number—the number of protons in the nucleus. This ordering exposes systematic patterns in chemical behavior, which are displayed in rows (periods) and columns (groups).
The atomic number uniquely identifies each element. For example, carbon’s atomic number is six, corresponding to its six protons.
A neutral atom has an equal number of electrons and protons. Thus, a neutral carbon atom contains six electrons and six protons.
Electrons fill energy shells from lowest to highest. Electrons in the outermost shell—valence electrons—govern how an atom bonds with others.
Rows on the table are called periods. All elements within a period share a valence shell; the number of valence electrons rises from left to right. When a shell is filled, a new period starts.
Columns, or groups, gather elements that share the same valence‑electron count, giving them similar chemical properties. For instance, the alkaline‑earth metals (Group 2) each possess two valence electrons and exhibit comparable reactivity.
By mapping atomic number and electron configuration onto a two‑dimensional grid, the periodic table provides a predictive framework for known and yet‑to‑be‑discovered elements.
