For the molecular formula HCNO many Lewis or Kekulé structures may be written. Try to write five or six such structures on a piece of paper. Remember that hydrogen can form only one covalent bond. Second row atoms such as carbon, nitrogen and oxygen can form more than one covalent bond, and try to achieve a valence shell electron octet by covalent sharing of electron pairs.
When you have finished, Click Here to see examples of nine structures of this kind.