Helium is placed in the 8th group of the periodic table despite not having 8 valence electrons because its electron configuration and behavior exhibit some similarities to elements in Group 8 (Group 18 in some periodic tables), also known as the noble gases.
Helium is unique among the elements because its electron configuration is 1s², meaning it has a completely filled 1s orbital with only 2 electrons. This configuration gives helium stability and makes it chemically unreactive, similar to the noble gases in Group 8 (such as neon, argon, and xenon).
The noble gases have full valence electron shells (with 8 electrons for most elements) and are generally chemically inert since their electron configurations are highly stable. Although helium doesn't have 8 valence electrons, it shares the characteristic of having a fully filled electron shell. Hence, it is placed in the same group as the noble gases.
The placement of elements in the periodic table is based on patterns and trends in their properties and electronic configurations, rather than solely on the number of valence electrons.