What physical property is most commonly associated with nonmetals and group 18 elements?

Nonmetals, particularly those in Group 18, also known as the noble gases, are commonly associated with being gases at room temperature. This characteristic is significant because noble gases like helium, neon, argon, krypton, xenon, and radon exist in a gaseous state under standard conditions.

The reason gases are a hallmark for nonmetals and noble gases is closely tied to their atomic structure. Nonmetals usually have higher electronegativities and bond in ways that often do not form metallic structures, contributing to their state as gases. Noble gases, in particular, are monatomic (composed of single atoms) and are inert due to their filled valence electron shells, further promoting their gaseous state under normal conditions.

In contrast, while some metals may exhibit properties like luster or conductivity, these characteristics are not applicable to nonmetals or noble gases. Similarly, melting points can vary widely among nonmetals and are not a definitive property that aligns with group 18 elements, as noble gases do not exhibit typical melting points as seen in solid materials. Thus, the most distinguishing and common association with nonmetals and group 18 elements is their existence as gases.