31
$\begingroup$

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?

$\endgroup$
  • 5
    $\begingroup$ You're basically comparing an intermolecular force with an intramolecular force. $\endgroup$ – Mehrdad May 25 '14 at 7:46
  • 1
    $\begingroup$ The problem with this "strength" is not well defined, and your teachers are using it in different sense. When bond dissociation energies are compared, ionic bonds can be pretty strong, to0: opentextbc.ca/chemistry/chapter/… $\endgroup$ – Greg Sep 17 '17 at 5:04
  • 1
    $\begingroup$ There is an additional confusion here as compounds consisting of covalent molecules don't make crystals based on covenant bonding: the crystals are formed from weaker dispersion forces (van der Waals forces) between the discrete molecules. This are definitely weaker than ionic bonds or the covalent bonds in network solids like diamond where every atom has a covalent bond to other atoms. $\endgroup$ – matt_black May 29 '18 at 15:06
18
$\begingroup$

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.

$\endgroup$
  • 1
    $\begingroup$ Your explanation doesn't seem to explain what you are saying. You explain using the fact that ionic compounds form lattice structures in the solid state but you then write that the boiling points of ionic salts are higher. In the liquid state, ionic bonds have already been broken. So when you mention about the boiling point of ionic salts, there is no relation to strength of the ionic bond. $\endgroup$ – Tan Yong Boon Aug 12 '17 at 12:22
  • $\begingroup$ @TanYongBoon, I used boiling points because I felt they are most comparable to the true energy required to break the individual ionic bonds (vs. melting points, at least). We tend to measure strength of ionic interactions by lattice energy, which is defined as the energy required per mol of ionic substance to convert the solid to constituent gaseous ions. This number, though, accounts for both sublimation energy AND individual bond enthalpy. Using boiling points, then, at least allows us to avoid comparing melting enthalpies and more directly compare bond enthalpies. $\endgroup$ – Shafter Oct 18 '18 at 8:12
  • $\begingroup$ @TanYongBoon of course the lattice nature of ionic compounds necessarily complicates the comparison, though. $\endgroup$ – Shafter Oct 18 '18 at 8:13
7
$\begingroup$

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.

$\endgroup$
  • 4
    $\begingroup$ As discussed in a different question, I respectfully disagree that ionic bonds are not as strong as covalent ones. $\endgroup$ – Geoff Hutchison Dec 18 '14 at 23:11
  • 2
    $\begingroup$ 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. $\endgroup$ – ron Dec 19 '14 at 16:15
  • 3
    $\begingroup$ 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. $\endgroup$ – Geoff Hutchison Dec 19 '14 at 16:40
4
$\begingroup$

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.

$\endgroup$
3
$\begingroup$

This turns out to be a nonsense question. Chemical bonds run the entire gamut from very strong to very weak as evidenced by the amount of energy required to break them. Trying to say that bonds which are either ionic or covalent are stronger is a big mistake, starting with the fact that "ionic" and "covalent" are merely the hypothetical extremes of the bonding continuum and can be considered "ideal" bonds. Real bonds lie along the continuum and have characteristics of both of the ideal bond types. Therefore, your original question has no place in the study of chemistry.

$\endgroup$
  • 2
    $\begingroup$ It is a returning issue due to the fact that basic courses (and the teachers of that) pretend to simply order bonds in terms of their strength. Add to this the fact that ionic bonds loose their strength in water (a working horse case for most if not all Chemists) and then you run into the fact that even here many argue that a covalent bond is in itself stronger than an ionic one. $\endgroup$ – Alchimista Sep 17 '17 at 15:27
2
$\begingroup$

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.

$\endgroup$
2
$\begingroup$

Don't confuse the strength of bonds with the strength of the forces that hold crystalline solids together

There is a reason why the lessons you learned from chemistry are different from the lessons learned in mineralogy: they are not talking about the same things.

The problem is that mineralogy the bonds they talk about are the bonds that hold crystals together but in in chemistry what is often talked about are the bonds that hold the molecules together not the crystals made from the molecules.

This distinction is important. The vast majority of crystals in chemistry consist of discrete molecules held together by weaker intermolecular forces (sometimes called van Der Waals bonds). These are fairly weak compared to covalent bonds and result in crystals that are weak and have low melting points. So a chemist might look at compounds where the bonding in the molecules is covalent and make the generalisation that they typically form crystals much weaker than metals or ionic compounds. But that is because the bonds that make the crystals are not covalent.

A mineralogist will mostly look at compounds which are not made of discrete molecules but are made from ionic lattices or covalent networks (or both). There are no molecules of diamond, the crystal is a network solid held together by an (almost) infinite array of C-C covalent bonds, as silica is held together by an infinite array of O-Si-O bonds. Other minerals are a mix of the two with many silicates containing, for example, sheets of O-Si-O structures with a variety of ions in between. So, to a mineralogist, covalent bonds look strong compared to other types of bond. Ionic bonds are strong but not as strong as purely covalent network solids.

And the problem is further complicate by definitions of strength that are too narrow. Are metals stronger or weaker than diamond-like structures? It depends what you mean by strength. Diamond is harder than any metal but it is also more brittle. If resilience to being struck with a sharp object matters, choose a ductile metal object over a diamond any day. This happens because the crystal structure in some metals can absorb energy by reorganising crystal defects rather than by shattering bonds (pretty much the only option in silica or diamond). So, in one sense, metals are stronger than covalent solids.

The overall lesson is to be careful about definitions. There is no good generalisation of crystal strength based on bond types. Be careful whether you are talking about the bonds within the components of the crystal (molecules) or the bonds that hold those components together (many "covalent" compounds consist of crystals where molecules are held together by much weaker forces). Don't forget that many minerals have both ionic and covalent bonding. And be specific about what you mean by "strength" (eg harness are resilience to impact are not the same thing).

$\endgroup$
2
$\begingroup$

I am not sure what is the consensus that has been arrived at by chemists around the world but I would just like to offer my two cents' worth on the issue. This question has always been a question which my teachers would always tackle when they teach chemical bonding and their answer has always been the same:

It is not fair to make a comparison since these bonds are ultimately very variable in terms of strength.

I do agree with that but allow me to provide my perspective to this issue.

The strength of covalent bonds in simple molecular substances (as well as those in giant network structures) can be easily determined. Thus, the bond energies of most covalent bonds are known well and they can be easily used for such comparisons of bond energy. However, the strength of ionic bonds and metallic bonds is not so clear-cut.

By definition, the ionic bond is the electrostatic force of attraction between positively and negatively charged ions in an ionic lattice while the metallic bond is the electrostatic force of attraction between the positively charged metal ions and the surrounding electrons.

In an ionic lattice, there are so many ions interacting electrostatically with each other. How then can the strength of the ionic bond be determined? The idea of lattice energy could be used but comparisons made using lattice energy would only be of any sense when we are comparing between ionic lattice. It cannot be used to compare with covalent bonds!

Consider the bond dissociation energy of $\ce {H-H}$ and the lattice energy of sodium chloride. The BDE of $\ce {H-H}$ is $\ce {+ 436 kJ/mol}$ while the lattice energy of $\ce {NaCl}$ is $\ce {+ 786 kJ/mol}$. Both are in terms of "per mole of something". But that "something" is different in each case. In the case of hydrogen, that "something" would be the $\ce {H-H}$ bond but in the case of the ionic compound $\ce {NaCl}$, that "something" is the $\ce {NaCl}$ formula unit. And that is not the same as "per mole of ionic bonds between $\ce {Na^+}$ and $\ce {Cl^-}$". The ionic bond strength is not so easily determined because each ion is in an electrostatic environment which is influenced by all the other ions around it. The same idea can be applied to metallic bonds.

In essence, my take is that there is no basis of fair comparison between metallic, ionic and covalent bonds in terms of their bond strengths.

$\endgroup$

protected by orthocresol Sep 17 '17 at 6:40

Thank you for your interest in this question. Because it has attracted low-quality or spam answers that had to be removed, posting an answer now requires 10 reputation on this site (the association bonus does not count).

Would you like to answer one of these unanswered questions instead?

Not the answer you're looking for? Browse other questions tagged or ask your own question.