Why is helium placed in group 18 even though it has an s² configuration?

Helium is placed in Group 18 because it shares similar properties with the noble gases, despite its s² configuration.

Helium is a unique element in the periodic table. It is the second lightest element and has an electron configuration of 1s², which is different from the other elements in Group 18 that have a configuration ending in p⁶. However, helium is placed in Group 18, the noble gases group, due to its chemical properties rather than its electron configuration.

The placement of elements in the periodic table is primarily based on their chemical properties, which are determined by the arrangement of electrons in their outermost shell. The noble gases, including helium, are characterised by their full electron shells and low reactivity. Helium has a full valence shell with two electrons, which makes it stable and unreactive, similar to the other noble gases. This is why, despite its s² configuration, helium is grouped with the noble gases.

Furthermore, helium's placement in Group 18 is also justified by its physical properties. Like other noble gases, helium is colourless, odourless, tasteless, and non-toxic. It also exists as a monatomic gas under standard conditions, which is another characteristic of noble gases.

In addition, helium's ionisation energy, which is the energy required to remove an electron from an atom, is comparable to that of other noble gases. This is another factor that contributes to its placement in Group 18.

In conclusion, while helium's electron configuration of 1s² differs from the p⁶ configuration of other noble gases, its chemical and physical properties align closely with those of Group 18 elements. Therefore, helium is placed in Group 18 of the periodic table.