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In mineralogy class, I was taught that metallic and ionic bonds are weaker than covalent bonds and that's why quartz and diamond have such a high hardness value. However, in organic chemistry class, I learned that covalent bonds are weaker than metallic and ionic bonds, thus organic substances have a much lower melting point than that of metals and ionic compounds.

What am I getting wrong? Are ionic and metallic bonds weaker than covalent bonds or not?

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You're basically comparing an intermolecular force with an intramolecular force. – Mehrdad May 25 '14 at 7:46
I did, yes. I understand now the principles. – Tamás May 25 '14 at 8:45
up vote 10 down vote accepted

Quartz and diamond are stronger substances because their molecules form network covalent structures. These structures form a lattice-like structure, much the same as ionic compounds.

This molecular network is also the reason that diamond and quartz form a crystalline structures, just like you'd see in ionic substances such as NaCl. Some other structures you might want to look into are Graphite and Graphene, which are both allotropes of carbon (allotropes are, simply put, different molecular arrangements of an element).

The network structure combines to make the substance stronger than normal covalent bonded substances.

So to answer your question, substances with standard covalent bonds seem to be weaker than those with ionic bonds because the ionic bonds tend to form a lattice structure, that makes them much stronger. You can see this in the fact that the boiling points of ionic salts are much higher than that of a covalent substance like water. However, when covalent bonds form network covalent structures, atoms combine to form a singular macromolecule that is much stronger than singular covalent bonds.

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What you learned in your mineralogy class was correct; bond strength decrease in the following order covalent > ionic > metallic. The reasoning for this is as follows. In covalent bonds such as those in methane and oxygen, the valence electrons are shared between the atoms involved in the bond and they (the electrons) spend most of their time in the region between the nuclei involved in the bond; this makes for a strong bond. In ionic materials such as sodium chloride, the electrons are donated from one (the electropositive) atom to the other (the electronegative) atom in order for the atoms to achieve a filled shell structure. The ionic atoms are attracted to one another through electrostatic attraction and the crystal lattices that are formed. The bonds formed through electrostatic attraction are not as strong as those formed from covalent sharing of electrons. Finally, in metals the outermost electrons are donated or "pooled" in the band structure that exists in metals. The electrons are free to travel great distances (hence the conductivity of metals) and serve as a glue to hold all of the positively charged metal nuclei together. So in the case of metals, there are no significant metal-metal bonds and these bonds are therefor the weakest.

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As discussed in a different question, I respectfully disagree that ionic bonds are not as strong as covalent ones. – Geoff Hutchison Dec 18 '14 at 23:11
In the above-referenced question you note that the largest covalent bond strength is 945 kJ/mol in $\ce{N2}$. While the ionic bond strength in $\ce{LiF}$, which should represent one of the stronger ionic bonds (see here) is only 577 kJ/mol. So at least when comparing the stronger covalent and ionic bonds, covalent bonds are stronger. Maybe you could provide BDE data for more average covalent and ionic bonds as well and we can see which are stronger in that realm. – ron Dec 19 '14 at 16:15
I also included some comments on other covalent bonds. But I think this is a fool's errand. The great insight of Pauling was that any bond with different atoms will have at least some ionic/electrostatic component. Indeed an article I referenced in my answer suggests many bonds are strongly covalent and strongly ionic. – Geoff Hutchison Dec 19 '14 at 16:40

Ionic and metallic bonds are weaker than covalent bonds. This is correct, it is why covalent crystal is much harder than ionic and metallic crystal/polycrystal.

The second statement is wrong because firstly melting point is not proportional to the strength of chemical bond. There are more factors such as flexibility of molecules. Boiling point is more proportional instead.

More important, the inter-particle forces to be compared among the organic compound vs ionic vs metallic compound is NOT among covalent bond vs ionic bond vs metallic bond. It is among intermolecular force (dipole-dipole, H-bond, Van der waals) vs ionic bond vs metallic bond. And the first one is much weaker than the second and third for sure. So the boiling point of organic compounds is much lower.

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It depends, because for covalent there are two types of bonds, network or molecular, or as I have also heard it be called, polar covalent and non polar covalent. But, network covalent consists of a vast network among the atoms and each one are connected, and they mostly made up of one element.

Take a diamond for example, it is only made up of carbon, but since the atoms are connected to each other and have not bonding between molecules, like something such as salt, which is an ionic bond, it is harder to break. However, if it were a molecular covalent bond, then the whole story is different, because they tend to be very weak bonds and easily broken like sugar or otherwise known as glucose, sucrose doesn't matter, it is still a covalent molecular bond because they have molecules while as a diamond is technically one big molecule.

But, since sugar has multiple the bond between each other molecule is weaker than the bonds between the elements themselves, then it is really weak.

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