What other intermoelcular forces can exist between neutral molecules? What kind of intermolecular forces can exist between neutral molecules?


My answer was London dispersion.

My logic was this: London dispersion is the only thing weaker than dipole-dipole, and as far as I am concerned two neutral molecules can't be attraced by dipole-dipole forces because you need a positive end on one and a negative one on the other: but there cannot be because, well, both are neutral, no?




  What are the intermolecular forces present in $\ce{CH3CH2NH2}$?


My answer was also London dispersion because it appears like this molecule is neutral.



Both answers are wrong. It appears I don't properly understand intermolecular forces. Why are my answers wrong?
This can be a bit tricky when you are first starting your journey into the world of intermolecular forces. What I like to look for first are what (if any) heteroatoms are present when the backbone of the molecule is composed of carbon atoms. 

In your case there is a nitrogen present and it is attached to 2 hydrogen atoms. This is a perfect example of a molecule that will exhibit hydrogen bonding as an intermolecular force. Hydrogen bonding, at least in my opinion, is the easiest to pick out. N, O, and F atoms bonded to Hydrogen are the only species in which this attractive force between molecules is observed. Now if you were to remove that $\ce{NH2}$ group and just had $\ce{CH3CH3}$ you would be correct. The only intermolecular force between the molecules would be London forces.