Which group on the periodic table is the least reactive? Why?

And why are they referred to as the Noble Gases?

The element's electron configuration can be used to determine an element's reactivity.

Elements like helium, neon, radon, and transition elements are examples of least reactive elements because they have a full outermost valence shell, meaning that they have eight electrons in the outer shell, which is the maximum number of electrons in the outer shell. This means that the element doesn't need to add any more electrons to its outer shell, nor does it need to remove any existing ones.

Elements with very few electrons in the outermost valence shell (i.e., elements with one electron), such as sodium, lithium, and potassium, are the most reactive because they will do anything to give up or remove their one electron in order to become more stable.

With their complete valence electron shells, noble gases minimize the tendency to gain or lose electrons.