Most nonmetallic elements exhibit significantly lower densities compared to metals, a difference rooted in their fundamental atomic structure and bonding. However, the nonmetal category includes elements in all three states of matter at standard conditions, leading to a wide range of densities that includes some notable, high-density exceptions. Understanding the typical low density of nonmetals requires examining how their atoms and molecules arrange themselves in space.
Understanding Nonmetals and Density
Nonmetals are elements that typically lack the properties associated with metals, such as high electrical and thermal conductivity, luster, and malleability. Found primarily on the right side of the periodic table, these elements tend to gain or share electrons in chemical reactions. Nonmetals can be identified by their general appearance, which is often dull, and by their physical state, as they exist as gases, liquids, and solids at room temperature.
Density is a measure of a substance’s mass per unit volume, effectively describing how tightly packed the matter is. A substance with low density has a relatively small amount of mass occupying a large volume. The nonmetals are overwhelmingly represented by the low-density end of this spectrum, though not without exceptions.
Structural Factors Leading to Low Density
The primary reason most nonmetals possess low density lies in their common physical states and the forces holding their particles together. At standard temperature and pressure, nearly half of the nonmetallic elements exist as gases, including oxygen, nitrogen, and the noble gases. Gases have extremely low densities because their molecules are widely spaced, occupying a large volume relative to their mass.
Even the solid and liquid nonmetals exhibit structures that result in less efficient packing than metals. Many nonmetals form discrete molecules, such as sulfur (S₈) or iodine (I₂), which are held together in the solid state by weak intermolecular forces. These forces are easily overcome and allow a significant amount of empty space to exist between the molecules. This loose arrangement contrasts sharply with the strong, uniform atomic packing seen in most metallic solids. The net result of these weak forces is a loosely packed structure, which translates directly to a low mass contained within a given volume.
Nonmetallic Elements That Defy the Trend
While low density is characteristic of the nonmetal group, the element carbon presents a significant challenge to this generalization through its different structural forms, known as allotropes. Diamond, one of carbon’s allotropes, is one of the densest naturally occurring materials known, with a density of about 3.51 grams per cubic centimeter. This high density is a result of a highly compact, three-dimensional crystal lattice where each carbon atom is bonded strongly to four neighbors.
Another carbon allotrope, graphite, is less dense than diamond, but still a dense solid. Its structure consists of carbon atoms arranged in flat layers, with the layers held together only by weak van der Waals forces. The relatively large space between these layers is what makes graphite less dense than diamond, even though the atoms within each layer are tightly bonded. Other solid nonmetals, such as iodine, also have relatively high densities. These examples illustrate that the specific bonding and structure of the solid nonmetal, rather than the element’s classification alone, determines its final density.
How Nonmetal Density Compares to Metals
Metals are generally much denser than nonmetals, creating a clear distinction between the two groups. The high density of metals is primarily due to the nature of metallic bonding, which involves a “sea” of shared electrons. This bonding allows the metal atoms to pack together very closely into highly efficient crystal lattice structures.
This dense, compact atomic arrangement, combined with the often higher atomic mass of many metallic elements, leads to a greater mass contained within a small volume. Elements like iron, copper, and especially osmium and iridium, which are among the densest elements, exemplify this trend. Conversely, while nonmetals occupy the low-density end of the spectrum, some light metals like lithium are less dense than even the densest nonmetals like diamond. Overall, however, the density spectrum is clearly divided, with nonmetals clustering toward the low end because of their molecular structure and weak particle attractions, and metals dominating the high end due to their tightly packed atomic lattices.