Which gas is described as highly reactive?

The answer highlights hydrogen as a highly reactive gas due to its chemical properties. Hydrogen has one electron in its outer shell, making it eager to bond with other elements to achieve a stable configuration, often resulting in the formation of compounds. This reactivity is particularly evident in its interactions with oxygen to form water, a process that releases a significant amount of energy. Additionally, hydrogen can react with various metals and non-metals, showcasing its tendency to readily engage in chemical reactions.

In contrast, other gases listed have different reactivity levels. Nitrogen, while it can participate in reactions (like in the formation of ammonia), is relatively inert under standard conditions due to the strength of the bond in its diatomic molecular form. Oxygen is reactive, especially in combustion and oxidation reactions, but it is less reactive than hydrogen in the context of forming bonds. Helium is a noble gas and is known for its very low reactivity, primarily because it has a full outer electron shell, making it stable and non-reactive in most scenarios.