Blue sky

A clear cloudless day-time sky is blue because molecules in the air scatter blue light from the sun more than they scatter red light.

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See:  http://yedda.com/questions/8621649071110/
The atmospheire obsorbes all colors and then scatters the light in such a way that you will see only blue.

 While the molecules that make up the earths atmosphere (76% N2, 21% O2 plus other trace gases) do not absorb light in the visible light spectrum, they do resonate with blue light. (wavelengths in the neighborhood of 450nm I think.) When the blue light hits the air molecules it they reflect light in different directions. The end result is that the sky is blue.

Similarly when the sun is setting, the light travels a long way through the atmosphere before it gets to you. When it finally does, all the blue light has been filtered (reflected) out, giving the sun it's red appearance.

I couldn't find on the web what you are saying about the air molecules resonate in blue light as the cause for blue sky.

Do you have a reference for this information?