Final answer:
Arsenic is the metalloid among the elements aluminum, arsenic, carbon, and helium. Metalloids have intermediate properties between metals and nonmetals, making them useful as semiconductors in electronics. The determination of an element as a metalloid can be challenging due to its mixed properties.
Step-by-step explanation:
Out of aluminum, arsenic, carbon, and helium, the element that is a metalloid is arsenic. A metalloid is a type of chemical element that has a combination of characteristics of both metals and nonmetals. Metalloids have properties that are in-between those of metals and nonmetals; for example, they have moderate heat and electrical conductivity. Arsenic, along with other metalloids such as boron, silicon, germanium, antimony, and tellurium, exhibits this intermediate behavior with an ability to act as semiconductors. This behavior is due to their electrons being more tightly bound than those of metal conductors, and it's why silicon is extensively used in electronics due to its semiconductor properties.
Boron silicates, which involve both boron and silicon, two metalloids, are used for applications such as heat-resistant glass and glass fiber insulation, showcasing the practical uses of metalloids. The difficulty in deciding whether or not an element is a metalloid is because their properties are not as clear-cut as those of typical metals or nonmetals, due to their intermediate electronegativity values and chemical behavior.