Along with Oxygen, which element is primarily responsible for the sky appearing blue?

The story behind the answer

Because our atmosphere is mostly nitrogen (~78%) with oxygen (~21%), sunlight passing through it is scattered mainly by these two molecules. Tiny N2 and O2 molecules preferentially scatter shorter (bluer) wavelengths much more than longer (red) ones, a process called Rayleigh scattering (intensity scales as 1/wavelength^4). The scattered blue light comes from all directions, making the sky look blue.

At sunrise and sunset, light travels a longer path, blue is scattered away, and reds dominate, hence orange/red skies. Larger particles (haze) scatter all colors more equally (Mie scattering), washing the blue out. Memory tip: "N2 + O2 = blue view."