Which group on the periodic table is the least reactive? Why?
Noble Gases (Group 18)
The reactivity of an element can be determined by looking at the electron configuration of the element.
The least reactive elements are those who have a full outermost valence shell ie they have 8 electrons in the outer shell so elements such as helium, neon, radon or the transition elements. Since they have 8 electrons in the outer shell and because the maximum number of electrons in the outer shell is 8 electrons, then the element doesn't need any extra electrons nor does it need to get rid of it.
The most reactive elements are those who have very few electrons in the outermost valence shell ie 1 electron so elements such as sodium, lithium, potassium. This is because the element will do anything to remove its 1 electron or to give away its 1 electron to make itself more stable. So it will be very reactive