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    when do van der waals forces increase and why?
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    With increasing relative atomic/molecular mass, because the number of electrons increases. You know that VDWs' forces are caused by an imbalance of electrons in the cloud of electrons on one atom which induces a dipole on another atom and so there is a weak attractive force between them. The bigger our atom/molecule, the more electrons and hence the more notable these imbalances and therefore the stronger the force. Think of for example Chlorine, it is a gas. Each atom has 17 electrons, and so a Cl2 molecule has 34 electrons. Imagine that 20 of these electrons are on one side of the electron cloud and 14 are on the other, the side with 20 is more -ve while the side with 14 is more +ve. The +ve side will induce a dipole on the neighbour molecule, causing the side of the electron cloud close to this +ve side to become -ve, and the same will happen with the -ve side. However in Iodine, I2, each molecule has 106 electrons. In one particular instant in time we may have 80 electrons on one side (-ve) and 26 on the other (+ve). This charge difference is much larger and hence the forces between these dipoles are stronger and that is why Iodine is a solid.

    In big organic molecules the number of contact points between neighbour molecules is important. Think of a pentane molecule, every carbon atom is available as a contact point with a neighbour molecule, while in 2,2-dimethylpropane (also 5 carbons), only 4 carbons are available as contact points, therefore pentane can establish more VDWs' forces with its neighbours, making it a liquid at r.t., while 2,2-dmp is a gas.

    Hope it helps!
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    When you say "contact points" can you ever get van der waals for carbon (e.g alkanes) because i would've thought only the hydrogens on the carbons resulted in van der waals. So for pentane there would be 12 contact points instead of 5?
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    Van der Waals forces are the weakest intermolecular forces, then permanent dipole-dipole, and finally hydrogen bonds are the strongest. Hydrogen bonds on happen if there is H, N, F or O
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    (Original post by Jennifer50)
    When you say "contact points" can you ever get van der waals for carbon (e.g alkanes) because i would've thought only the hydrogens on the carbons resulted in van der waals. So for pentane there would be 12 contact points instead of 5?
    Oh sorry, true. I was thinking of the skeletal formula. But you don't really need to specify because pictures always show the skeletal or structural formula so just say that the higher the number of contact points the stronger the VDWs' forces between the molecules.
 
 
 
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