dolphins123
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why do the intermolecular forces within a simple covelant molecules increase when its size increases?
e.g. methane has a lower boiling point than ethane
or iodine has a higher boiling point than chlorine
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username5137494
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(Original post by dolphins123)
why do the intermolecular forces within a simple covelant molecules increase when its size increases?
e.g. methane has a lower boiling point than ethane
or iodine has a higher boiling point than chlorine
Hey dolphins123, thank you for the question.
Define "intermolecular forces increase"?
Also, you need to be more explicit when you say "its size increases".
I am assuming based on your examples that you are referring to MOLECULAR MASS.
Now I really don't want to confuse you because the answer lies in A-Level knowledge, nothing required for you GCSE spec :-)
But here's a few points to consider:
1. Higher molecular mass = more protons = bigger atomic radius (in general) and- IMPORTANTLY- more ELECTRONS.
2. According to the wave function theory, electrons move randomly about an atom in what can be modelled as probability clouds, and sometimes they will be all around roughly the same phase of their respective orbitals.
3. This means that they will make THAT side of the electron slightly negatively charged
4. But the opposite side will barely have any electrons so it will be slightly positively charged
5. So any atoms in the neighbourhood which happen to have an area of electron density (due to random movement of electrons, exactly the same phenomenon! ) will end up being attracted to that atom.
6. This explains why atoms with larger size (i.e. electrons) feel this sort of intermolecular attraction more- this is also known as "induced dipole interaction" since what the electron density gap creates is essentially a temporary dipole (like magnets).

But hey,
EDIT: I think you misunderstood something. It's usually the fact that there are MORE BONDS to break that determines the increase in boiling point in alkanes. Nothing to do with intermolecular forces being stronger. Just more of them :P

Good luck with your exams!
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dolphins123
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(Original post by Avtaras)
But hey,
EDIT: I think you misunderstood something. It's usually the fact that there are MORE BONDS to break that determines the increase in boiling point in alkanes. Nothing to do with intermolecular forces being stronger. Just more of them :P

Good luck with your exams!
hey, thank u for the response i have much clearer understanding of the relationship between intermolecular forces and melting points.
however i'm a bit confused about how the number of bonds determine the melting/boiling point, because in my textbook it states that 'in simple covalent structures, there are weak intermolecular forces between neighboring molecules, so it has a low boiling point' but for giant covalent lattices like diamond, there are no intermolecular forces so why is the melting/boiling point high?
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username5137494
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(Original post by dolphins123)
hey, thank u for the response i have much clearer understanding of the relationship between intermolecular forces and melting points.
however i'm a bit confused about how the number of bonds determine the melting/boiling point, because in my textbook it states that 'in simple covalent structures, there are weak intermolecular forces between neighboring molecules, so it has a low boiling point' but for giant covalent lattices like diamond, there are no intermolecular forces so why is the melting/boiling point high?
Okay listen
Yes ur right, the reason why simple covalent bonds are weak is because of weak intermolecular bonds, not anything else. Electrostatic attraction in those bonds is VERY STRONG, so it’s just the bond with other covalent molecules which is weak.

You got confused. Both bond energies and intermolecular bonds account for high boiling/melting points. So both my point and yours are valid.

But simple covalent bonds’ weakness is more to do with intermolecular attraction forces being weak since the electrons only feel electromagnetic induction to neighbouring atoms momentarily before returning to their random positions. There is no source of pull of electrons (polarity) in most covalent bonds.

You’re right that more bonds means more energy required to break those bonds when you melt/boil it, but that’s less important in this particular example.
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