What type of gas is highly reactive due to lacking one electron?

Halogen gases are highly reactive because they are one electron short of having a complete outer shell, which is stable. This characteristic drives them to form bonds with other elements, often by gaining an electron to achieve a full valence shell, typically consisting of eight electrons. Their reactivity is notably high in comparison to other groups of elements due to their strong tendency to attract this additional electron.

In contrast, noble gases are inert and have complete outer electron shells, making them largely nonreactive. Inert gases refer to the same group of elements categorized as noble gases, further underscoring their stability and lack of reactivity. Hydrogen gas, while reactive under certain conditions, does not possess the same intrinsic reactivity as halogens due to its unique electron structure and does not belong to the halogen category. Therefore, the nature of halogen gases, specifically their one electron deficiency and high reactivity, establishes them as the correct answer in this scenario.