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Why do simple molecular substances have weak intermolecular forces and why do giant covalent substances have strong intermolecular forces?

I understand that in simple molecular substances the atoms within a molecule are held together by strong covalent bonds, but the intermolecular forces between molecules are weak.

But I don't see what makes giant covalent substances have stronger intermolecular forces? What makes them stronger and have higher melting points?

I thought I understood but I think I must have gone wrong somewhere and now I am very confused. Help would be much appreciated. Also please explain simply, chemistry does not come naturally to me so I might get even more confused.

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    $\begingroup$ Are you sure you're not comparing apples to oranges? Ask yourself this: in a giant covalent substance, what does an intermolecular force mean? Is there more than one molecule to interact with one another? Think about the differences between intermolecular and intramolecular forces. $\endgroup$ Commented Jan 7, 2015 at 15:25
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    $\begingroup$ Are you defining "giant covalent substances" as larger molecules, such as octane in comparison to methane? Or using the phrase to mean large covalent networks, such as diamond? $\endgroup$
    – venture
    Commented Jan 7, 2015 at 19:08

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In simple molecules the intermolecular forces are, in order of increasing strength, dispersion forces, permanent dipole interactions and hydrogen bonding (which has significant covalent character but is generally considered to be an intermolecular force).

By contrast giant covalent repeating structures such as diamond and $\ce{SiO2}$ are not molecular in the same sense as they can theoretically be infinitely large. Therefore they do not really have intermolecular forces but they are simply held together by covalent bonds between the atoms in the structure.

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  • $\begingroup$ So giant covalent structures don't have intermolecular forces because they aren't molecules? $\endgroup$
    – francesca
    Commented Jan 7, 2015 at 16:02
  • $\begingroup$ @francesca it depends on how you define molecules but very large strcutures like diamond, graphite, $\ce{SiO2}$ etc. are so large that intermolecular forces are not really relevant because there may only be one (or a few) 'molecules' $\endgroup$
    – bon
    Commented Jan 7, 2015 at 18:38
  • $\begingroup$ You are forgetting the vast number [>10^23] of atoms involved a g.at.wt. of diamond or graphene possibly has 10^10 to 10^15 "molecules". Just a guess. $\endgroup$
    – jimchmst
    Commented Sep 30, 2022 at 17:26
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Molecules are made of fixed numbers of atoms joined together by covalent bonds, and can range from the very small (even down to single atoms, as in the noble gases) to the very large (as in polymers, proteins or even DNA).

The covalent bonds holding the molecules together are very strong, but these are largely irrelevant to the physical properties of the substance. Physical properties (melting points, boiling points, solubility in water,...) are governed by the intermolecular forces - forces attracting one molecule to its neighbors - van der Waals attractions or hydrogen bonds.

Molecular substances tend to be gases, liquids or low melting point solids, because the intermolecular forces of attraction are comparatively weak. You don't have to break any covalent bonds in order to melt or boil a molecular substance.

The value of the melting or boiling point will depend on the strength of the intermolecular forces. The presence of hydrogen bonding will lift the melting and boiling points. The larger the molecule the more van der Waals attractions are possible - and those will also need more energy to break.

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Two factors matter: the nature of the bonds and the scale of the interactions

The simple case is that, for many giant covalent substances, all the bonds are strong covalent bonds. Diamond, where all the carbons in the substance are connected to each other by strong covalent bonds, is a good example.

The more complex case relates to how strong the weak interactions between individual molecules (van der Waals forces) are in molecules of different size. Iodine, a substance containing small molecules containing two iodine atoms, is volatile because it has weak forces between the molecules. But polyethylene, a polymer is a fairly solid and non volatile substance, contains long strands of alkane-like chains held together by the same forces.

But the strength of those weak forces between the components making up the substance, depend on the surface area of the components. Iodine molecules have a small surface area, so the forces are weak; polyethylene chains have a large surface area, so the attractive forces are strong. This is also why geckos can walk up walls: same forces at play but the structure of their fingers and toes creates huge surface area maximising the overall strength of the weak interactions.

The surface area also explains why the shape of the molecule matters. Compact molecules have smaller forces than their isomers of the same size that are less compact. So the compact 2,2 dimethyl butane (sometimes called neopentane, C(CH3)4) is far more volatile than its isomer pentane, a 5-carbon long chain.

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  • $\begingroup$ Far more volatile? maybe more volatile. Same mass not the same size. $\endgroup$
    – jimchmst
    Commented Sep 30, 2022 at 17:43
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In covalent molecules, there’s a theory that the bigger the molecule, the higher the melting point and boiling point.
However, the main reason why giant covalent structures have high melting and boiling point is that it is the strong covalent bond that must be overcome in order to melt or boil the giant covalent structure.
Let me use graphite as an example, although there is weak intermolecular bonds between the layers of carbon atom (which is actually graphene). However, breaking the weak intermolecular bonds between layers of carbon atom will only allow layers of carbon atom to slide over each other, which make graphite slippery and soft. In order to melt or boil graphite, you have to break the strong covalent bonds.
In contrast, for simple molecular structures, the reason why it have low melting and boiling point is because it is the weak intermolecular forces that must be overcome in order to melt or boil it.

I understand it’s so complicated, but as long as you review it for a few times, you will remember it.

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  • $\begingroup$ Is it a theory or a simple observation? Evaporation and decomposition are not the same. Evaporation returns the same molecular composition on condensation. Decomposition in elements seems to do the same but the vapor composition is not molecularly the same as the condensed phase. That most elements have only one common allotrope it seems that they might be evaporating. If allotropes are possible they can be interconverted. Taking carbon as the example decompose graphene recompose diamond. $\endgroup$
    – jimchmst
    Commented Sep 30, 2022 at 23:57

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