"The
strength of a
covalent bond is measured by its
bond dissociation
energy, that is, the amount of
energy required to break that particular
bond in a mole of molecules. Multiple
bonds are stronger than
single bonds between the
same atoms."
With Diamond, Each carbon can form 4 other carbon bonds to one another and if we theoretically had an infinite volume, It could go on for ever.
With Oxygen, All you need is 2 oxygen atoms and that is it. They form a double bond to one another and nothing else.
Here is a visual representation:
Now Diamond could continue bonding forever and ever and ever...............................
Oxygen, It only forms a single bond with another Oxygen and
that is it.I'm sure you're aware of Intermolecular forces right? If you want to melt a Covalent compound, you would have to break the London Dispersion Forces (Van der waal forces) between the molecules.
The diamond I drew has an Mr of 264
The oxygen has an Mr of 32. (Oxygen wouldn't have Dipole-dipole as the polarities cancel due to it being Linear)
Which one would have more points of Interaction for Van der waal forces?
That's why Diamond has a higher Melting point.