[matea]

i wonder why is NO₂ coloured and NO is colorless, both compounds have unpaired electron,,,,
please help

[DevaDevil]

It's about the difference of the filled and unfilled Molecular Orbitals (MO's) in the compound. A coloured compound has a difference in energy between a filled and unfilled orbital corresponding to visible light.
The difference in energy levels between the highest (doubly) filled MO and lowest non-filled MO in NO is higher than in NO₂ making it absorb UV (that we cant see).

Now it's been some time since I last messed with MO-diagrams but maybe someone here has the perfect program to show this? And please do correct me if I am wrong in the drawings:

I tried to show it in the following schetches, but by no means are they perfect  The energies are not in schale, but it gives an indication
NO
NO₂

The blue arrow indicates the electron transition that takes place and clearly shows the lower energy is needed in NO₂. I have no data for the exact energy levels available now, but since NO₂ is colored, the transition must be withing the energy levels of visible light.

Hope this helps,
Dennis

[matea]

tnx:)
i would be very thankful if u could say the exact values if u find them,,cause i couldn't find them,,,
matea