Cognitive Reflection Test (CRT) Practice 2026 - Free Cognitive Reflection Test Practice Questions and Study Guide

Helium is recognized as a noble gas due to its unique properties and position in the periodic table. Noble gases are characterized by their lack of reactivity, which stems from having a complete set of valence electrons. Helium, with its atomic number 2, has two electrons that fill its outer shell, rendering it stable and nonreactive under standard conditions. This stability is a defining feature of noble gases, including Neon, Argon, Krypton, Xenon, and Radon, all of which similarly demonstrate minimal chemical interactions with other elements.

In contrast, the other substances mentioned, such as oxygen, carbon dioxide, and hydrogen, are not categorized as noble gases. Oxygen and hydrogen are diatomic molecules essential for life and engage in various chemical reactions. Carbon dioxide, a compound formed from carbon and oxygen, is also reactive and involved in processes like photosynthesis and respiration. Thus, helium stands out as the only noble gas among the choices given, reflecting its inert characteristics in the context of chemical interactions.