the units digit of the group number matches the number of valence electrons. (Helium is the sole exception.)

Helium is an exception: despite having a 1s² configuration with two valence electrons, and thus having some similarities with the alkaline earth metals with their ns² valence configurations, its shell is completely full and hence it is chemically very inert and is usually placed in group 18 with the other noble gases.

Valence shell

The valence shell is the set of orbitals

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