Hydrogen has one electron, helium has two electrons, and lithium has three electrons. Hydrogen and lithium are both reactive, but helium is nonreactive. How could the quantum mechanical model of the atom explain this pattern of behavior?

Well depending on all the factors of this element, you’d have to look at the odor, color, taste, reactivity, e.c.t. Odor and Color are not the biggest factor but reactivity is what we are looking at so, looking at the quantum mechanical model, we would see that Helium is a unique element with its own characteristics. Each of these characteristics effect the reactivity of each element, which Hydrogen and Lithium, both have similar properties which relate back to why they are both reactive, since they share similar elemental properties.