The term for the gas that gives the blue color to the sky is Rayleigh scattering, primarily caused by the atmospheric nitrogen and oxygen. When sunlight enters the Earth's atmosphere, it interacts with these gases, scattering shorter wavelengths, particularly blue light, in all directions. This scattering results in the blue appearance of the sky. As the sun sets or rises, the light passes through more atmosphere, scattering shorter wavelengths and allowing longer wavelengths like red and orange to dominate, resulting in vibrant sunsets.