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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. $\endgroup$ – Nicolau Saker Neto Jan 7 '15 at 15:25
  • $\begingroup$ 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 Jan 7 '15 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 Jan 7 '15 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 Jan 7 '15 at 18:38
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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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