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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?

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It sounds like you are confusing polarity with formal charge. Polarity is an uneven distribution of electron density within a molecule. This uneven distribution can occur for several reasons, but the most common is difference in electronegativity between atoms.

Consider the $\ce{O-H}$ bond. The electronegativy of an oxygen atom is 3.44 and that of a hydrogen atom is 2.20. The difference (1.24) is quite large given the scale commonly only goes to 4.0. Thus, the $\ce{O-H}$ bond is polarized toward the oxygen atom. The oxygen atom has a stronger pull on the electron pair than the hydrogen atom. More electron density is around the oxygen atom than the hydrogen atom. Or, if you prefer the other interpretations, the electron pair is closer to the oxygen atom or spends more time around the oxygen atom.

Regardless, there is an uneven sharing of electrons, and the oxygen atom, has more. The oxygen atom thus can be said to have a partial negative charge to represent its overabundance of electron density. This partial charge is not a full formal charge, but rather a shorthand notation to help us remember that the oxygen atom has more than an even share of the electron density in the bond. We represent these partial charges with $\mathrm{\delta^{-}}$. Consider the notation on a water molecule:

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Because of these uneven distribution of electrons, water (which is a neutral molecule) has a permanent dipole. These dipoles attract each other.

The type of force that exists between ions is called the ionic interaction.

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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.

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