Which of the following elements is a halogen?

Bromine is classified as a halogen because it belongs to Group 17 of the periodic table, which is known as the halogens. This group includes elements such as fluorine, chlorine, bromine, iodine, and astatine. Halogens are characterized by their tendency to form salts when they react with metals and their diatomic molecular form in their elemental state. Bromine, in particular, is unique among the halogens as it is the only one that exists as a liquid at room temperature.

In contrast, helium and argon are noble gases found in Group 18. Noble gases are characterized by their full valence electron shells, making them very stable and largely unreactive. Calcium, on the other hand, is an alkaline earth metal located in Group 2 of the periodic table, which is composed of metals that readily lose electrons to form cations. These differences in group classification and chemical properties highlight why bromine stands out as the correct choice among the given options.