Why is helium placed in group 18 on the periodic table?

Helium is placed in Group 18 with the other noble gases because of its chemical properties, even though its electron configuration might suggest it belongs in Group 2. With only two electrons in the $1s$ orbital $(1s^2),$ helium has a completely filled valence shell, making it extraordinarily stable and chemically inert. This complete valence shell gives helium the same fundamental characteristic as all other noble gases: it has no tendency to gain, lose, or share electrons under normal conditions. Like neon, argon, krypton, and xenon, helium exists as monatomic gas and forms no known stable compounds under standard conditions, making its chemical behavior identical to the other Group 18 elements.

The periodic table is organized primarily by chemical properties rather than strict electron configuration patterns. If helium were placed in Group 2 above beryllium based solely on having two valence electrons, it would be grouped with the alkaline earth metals, which are highly reactive metals that readily lose two electrons to form $2+$ cations. This would be completely misleading since helium shares none of these properties. Instead, helium's placement in Group 18 correctly indicates that it is an unreactive noble gas.

The key insight is that for helium, two electrons represent a full valence shell (the first shell only holds two electrons), while for beryllium, two electrons represent an incomplete second shell that can hold eight. This distinction between a full shell and simply having two electrons is crucial, and it is why helium rightfully belongs with the noble gases despite its unique electron configuration.