Methanamide, HCONH2, is a planar molecule. All the bond angles in the molecule are approximately 120 degree.The C-N bond length measured is about half way between that typical for C-N single bonds and C=N double bonds. Please explain these observations.

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Methanamide (HCONH2) is also called formamide.  The standard Lewis structure notation involves a single bond between the hydrogen and the carbon, a double bond between the carbon and the oxygen, a single bond between the carbon and nitrogen, and single bonds between the nitrogen and both hydrogens.  But there is another Lewis structure that is also applicable here where the carbon and oxygen have a single bond and the carbon and nitrogen have a double bond.  These two structures are called resonance structures.  Don't think of is as constantly alternating between the two structures.  It is more correct to think of the true structure as a hybrid of the two separate structures.  That is why all of the bond angles are 120 degrees and the carbon nitrogen bond length is between single and double, since the two central atoms (C and N) both have characteristics of being sp2 trigonal planar.

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