For one of my chemistry projects, I have to build a 3D model of urea, with 3 urea molecules bonded to eachother (via hydrogen bonding). I was just wondering how you know the shape of the hydrogen bond that takes place?? Ie. for water, the hydrogen bonding makes a tetrahedron shape, whats the shape/angle made by urea molecules?? Also, if anyone can explain why Perchloroethylene (Cl2C=CCl2) is effective for dry cleaning (i.e. the polarity, bonding, etc) that would be really appreciated thanks!