Helium is indeed in group 18 of the periodic table, which is known as the noble gases. While all noble gases are characterized by having full outer electron shells and being generally nonreactive, helium has some distinct differences compared to the other members of this group:
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Electron Configuration: Helium has an atomic number of 2 and has a full outer electron shell (1s²), which is complete with just 2 electrons. In contrast, other noble gases, such as neon (atomic number 10) and argon (atomic number 18), have more complex electron configurations with full outer shells that include additional subshells (e.g., 2s² 2p⁶ for neon).
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Size and Atomic Radius: Helium is the smallest noble gas, both in terms of atomic size and atomic radius. This smaller size contributes to its different physical properties compared to the other noble gases.
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Boiling and Melting Points: Helium has significantly lower boiling and melting points compared to other noble gases. It remains a liquid at very low temperatures and does not solidify under normal atmospheric pressure, which is not the case for other noble gases.
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Quantum Behavior: Helium exhibits unique quantum properties, such as superfluidity at very low temperatures. This is not observed in heavier noble gases.
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Reactivity Under Extreme Conditions: While noble gases are generally inert, helium can show some reactivity under extreme conditions, such as in high-energy environments (e.g., in stars), although it is still much less reactive than many other elements.
These differences make helium an interesting element in group 18, and it is often treated somewhat separately from the other noble gases in discussions of chemistry and physics.