The periodic table systematically organizes all known chemical elements based on increasing atomic number and recurring chemical properties. This arrangement allows scientists to predict an element’s behavior simply by its position. Elements are broadly classified into three main groups: metals, nonmetals, and metalloids. While most elements are metals, nonmetals exhibit properties far different from their metallic counterparts, particularly in how they bond with other atoms.
The Total Number of Nonmetals
The most commonly accepted count places the number of nonmetals at 17 elements. This specific number includes the six noble gases, which are chemically distinct but still categorized as nonmetals due to their physical properties. These 17 elements possess high electronegativity, meaning they tend to attract electrons in chemical bonds, and they are typically poor conductors of heat and electricity.
This group of 17 nonmetals includes Hydrogen (H), Carbon (C), Nitrogen (N), Oxygen (O), Phosphorus (P), Sulfur (S), and Selenium (Se). The halogens—Fluorine (F), Chlorine (Cl), Bromine (Br), and Iodine (I)—are also counted. The list is completed by the six noble gases: Helium (He), Neon (Ne), Argon (Ar), Krypton (Kr), Xenon (Xe), and Radon (Rn). These elements commonly form covalent bonds and lack metallic luster, although some, like solid Iodine, can appear somewhat shiny.
Identifying Nonmetals on the Periodic Table
Nonmetals are predominantly clustered on the right side of the periodic table, occupying the upper-right corner, with one notable exception. Their location is defined by a bold, step-like boundary, often called the “staircase line,” which separates them from the metals to the left. This separation reflects the fundamental shift from electropositive metals to electronegative nonmetals. Moving across a period, elements gain more nonmetallic character as their atoms become smaller and hold their electrons more tightly.
The nonmetals are commonly organized into three functional subgroups based on their reactivity and position. The first group includes unclassified nonmetals like Oxygen and Carbon. The second subgroup is Group 17, known as the halogens, which are highly reactive elements that readily gain a single electron to achieve a stable electron configuration. The third subgroup, Group 18, consists of the noble gases, which are unique for their full outer electron shells and chemical inertness.
The reactive nonmetals and halogens contrast sharply with the noble gases, which are so stable they rarely participate in chemical reactions under normal conditions. This arrangement highlights a trend: elements become increasingly nonmetallic and chemically stable as you move from the left to the far right of the table. The only nonmetal found far away from this corner is Hydrogen, which sits alone in the top left, reflecting its single valence electron and singular nature.
Why the Count Can Sometimes Vary
The precise number of nonmetals is often debated due to a lack of a single, universally accepted chemical definition for all element categories. The primary source of variation lies in the classification of the metalloids, which sit directly on the staircase line. Metalloids, such as Boron (B), Silicon (Si), and Germanium (Ge), exhibit intermediate properties; they may conduct electricity under certain conditions but are brittle like nonmetals.
Depending on the textbook or scientific organization, metalloids may sometimes be counted with nonmetals, pushing the total count to 20 or more. Different classification systems place the boundary differently, leading to small discrepancies. For instance, some chemists might include Astatine (At) as a nonmetal, while others treat it as a metalloid due to its uncertain properties.
A second factor introducing variation is the placement and nature of Hydrogen (H). Although it sits in Group 1 with the alkali metals, Hydrogen is undeniably a nonmetal under normal conditions, existing as a diatomic gas. Some specialized classifications exclude Hydrogen from any group or place it in its own unique position, which affects the overall count if a system strictly counts elements by group. These boundary issues illustrate that the periodic table is a scientific model, and the lines drawn between element types are based on a spectrum of observable properties, not absolute divisions.