Which elements are typically poor conductors of heat and electricity, and are generally gases or brittle solids?

Nonmetals are typically poor conductors of heat and electricity, distinguishing them from metals, which are known for their excellent conductivity. This characteristic results from the electronic structure of nonmetals, which generally have more valence electrons that are not as free to move compared to the electrons in metals. As a consequence, nonmetals often exhibit properties such as brittleness in solid forms and a tendency to be gases at room temperature.

In terms of their physical state, many nonmetals exist as gases (like oxygen and nitrogen), while those that are solids (such as sulfur and phosphorus) are generally brittle rather than malleable or ductile. This lack of malleability further reinforces their conceptual distinction from metals. Nonmetals also tend to form anions during chemical reactions, indicating their higher electronegativity compared to metals.

Halogens and noble gases, while also classified as nonmetals, have more specific characteristics. Halogens are highly reactive and exist in various states (gases and solids) at room temperature, while noble gases are inert and are primarily gases. However, it is the broader category of nonmetals that best captures the characteristics described in the question regarding conductivity and physical state.