how do you determine if an atom is polar or non-polar? I\'ve never been good at

Question

how do you determine if an atom is polar or non-polar? I've never been good at determining intermolecular forces and really wanna be good at it. I know London dispersion exists everywhere, but the other ones such as dipole-dipole, dipole-induced...how do you determine that if a compound is given? Such as CHCl3in CH3-CH2-OH. Totally confused. The answer would be Dipole-dipole but why? I know it is related with polarity, is the best way to determine polarity drawing the molecule out? Sorry for asking so much questions, hope someone could help me on this intermolecular part.

Explanation / Answer

Hello,

I think you are really close to understanding the concept So I hope I can try and clear things up for you.

First, the polarity of a molecule is affected by the electronegativity of each individual atom. Electronegativity determines how strongly an atom attracts and holds onto electrons. Electronegativity increases across the periods and up the groups, so F is the most electronegative atom.

Now to polarity, when a molecule has a strong electronegative atom, like Cl, attached to a less electronegative atom, like C, The electronegative atom pulls the electron density toward itself. This creates a partial negative charge on the electronegative atom and a partial positive charge on the other atom. A dipole is created (2 poles, the positive and negative) and the molecule is polar!

Now for the Intermolecular Forces!

van der Waals forces come in 2 types. The first is called a dispersion force (aka London dispersion force) The fundamental principle behind these forces is that electrons moving around the electron cloud of any molecule may at some point in time randomly create a dipole, this is known as an instantaneous dipole. Now any molecule close to the instantaneous diploe will automatically create a dipole as well, because it's electrons are repelled by the partial negative charge. Because this dipole is caused by the instantaneous dipole it is called the induced dipole. These molecules are arranged so that the positive end of one molecule is close to the negative end of the other. And this + and - attraction is the Dispersion force! Note that this is the weakest intermolecular force.

Next is the second kind of can der Waal force known as the Dipole-Dipole force. This force occurs between molecules that are polar. Because the molecule is polar it has a permanent dipole, therefore the + and - ends of the molecules line up thte same as before, with + end of one molecule close to the - end of the other. Because the dipoles are permanent this force is stronger yet.

Now let's go back to polarity for a second. You already know that polarity has to do with electronegativity, but in order for a dipole to occur there must be a net pull in a certain direction because Electronegativity is a vector quantity, So for example if we had O=C=O, this is not polar because the electronegativity vectors cancel each other out, 1 pulls left and 1 pulls right. But ClCH3 is polar because the electrons are pulled away from the carbon atom.

The last type of intermolecular force is Hydrogen Bonding! Perhaps the most important! (If you do not need to know about this yet then skip this paragraph) H-bonding occurs when Hydrogen is covalently bonded to an electronegative nonmetal atom. The pull of the electronegative atom basically makes the H a postive charge. The H is attracted to the molecules with the small strong electronegative atoms, N,O and F, espically their lone pairs! (technically it is seen in Cl and S too, just not as strong) This is the strongest intermolecular force and is seen in water.

So what does this all mean? Well the stronger the intermolecuar forces are the stronger molecules are held together, which affects properties such as melting points and boiling points.

I hope this helped you better understand the intermolecular forces.

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