Helium gas is chemically inactive because of its complete and stable electron configuration. Helium has two protons in its nucleus, which are balanced by two electrons orbiting the nucleus. These electrons fill the first electron shell, which is the innermost and lowest energy level of the atom.

The electron configuration of helium is 1s^2, which means that both of its electrons are in the 1s orbital. The 1s orbital is spherically symmetrical and has a very low energy level. This means that the electrons in the 1s orbital are very tightly bound to the nucleus and are not easily removed or excited.

As a result of its stable electron configuration, helium gas does not react with other elements to form chemical compounds. This is because helium does not have any unpaired electrons that can participate in chemical reactions. All of its electrons are paired and are in the lowest energy level possible, so there is no driving force for helium to react with other elements.

In addition, helium gas has a very high ionization energy. This means that it takes a lot of energy to remove an electron from a helium atom. This also contributes to helium's chemical inactivity, as it makes it difficult for helium to form chemical bonds with other elements.

Helium gas is also a noble gas, which means that it is a non-metallic element that does not react with other elements under normal conditions. Noble gases are all characterized by their complete and stable electron configurations, which makes them chemically inert.