Forces of Attraction

If H2CO was bonded to itself, the forces of attraction involved would be:
London Dispersion Forces
   Very weak attraction forces due to the constant movement of electrons around atoms.


Dipole - Dipole Attraction Forces
   An electro-static attraction between the positive end of a molecule and the negative end of another. [represented by a dotted line]
       O = C -- H - - - - - - O = C -- H
               |                                 |
              H                               H


Hydrogen Bond 
   A covalent bond temporarily forms between a hydrogen (H) and either a nitrogen (N), oxygen (O), or a fluorine (F). [represented by a solid line]
       O = C -- H --------- O = C -- H
               |                                 |
              H                               H