Final answer:
Arsenic is the element that is not a nonmetal among the options listed. Nitrogen, helium, and oxygen are nonmetals, while arsenic is a metalloid with both metallic and nonmetallic properties.
Step-by-step explanation:
All of the following elements are nonmetals EXCEPT one: arsenic, nitrogen, helium, and oxygen. In the context of the periodic table, nonmetals are characterized by being poor conductors of heat and electricity, and they are typically found in the top-right corner of the periodic table, with the exception of hydrogen. They can exist in various states of matter: gas, liquid, or solid, and they have a greater variety of properties compared to metals.
While nitrogen (N), helium (He), and oxygen (O) are all nonmetals, arsenic is not. Arsenic (As) is actually categorized as a metalloid, which is a type of element that has a mixture of metallic and nonmetallic properties. Metalloids like arsenic are found along the diagonal line between the metals and nonmetals in the periodic table, which includes elements such as boron, silicon, germanium, antimony, and tellurium.
Therefore, the correct answer to the question is that arsenic is the element that is not a nonmetal.