Chemical elements that behave sometimes like metals and sometimes like nonmetals are often called ___.

The term that describes chemical elements which exhibit properties of both metals and nonmetals is metalloids. Metalloids typically possess a mixed set of characteristics; for example, they can be brittle like nonmetals but also conduct electricity somewhat like metals. This dual behavior allows them to serve various roles in chemical reactions and applications, often being used as semiconductors in electronics. Understanding the nature of metalloids is essential for grasping the broader categories of elements in the periodic table as well as their interactions.

The other options do not fit this definition. Nonmetals are distinct from metals and do not possess the metallic properties that metalloids do. Alloys refer specifically to mixtures of metals, rather than individual elements with properties that aren't strictly metallic or nonmetallic. Compounds are substances formed when two or more different elements chemically bond together, which does not relate to the classification of elements based on their metallic or nonmetallic behaviors. Thus, metalloids remain the correct answer, accurately capturing this unique characterization.