Ionic bond vs covalent bond.

  1. As i know chemical bonding can be considered as a spectrum of bond ranging from covalent to ionic. Now can we say that ionic bond is harder than covalent bond?? Since covalent bond is a result of attractive n repulsive bt ionic only attractive .
    If it does, instead of bond length factor, HF bond is stronger than HI bond can we reason that the covalent HF bond contains a higher ionic character so a stronger bond than HI bond??? Thanks for your help. :)
     
  2. jcsd
  3. No you can't say an ionic bond is stronger than a covalent bond. The strongest substance on earth, diamond, is entirely covalently bonded. There are too many factors that go into the strength of a bond to make any kind of generalization like this.
     
  4. AGNuke

    AGNuke 456
    Gold Member

    You forgot Carbon Monoxide. Its bonding is even strong than N(Triple Bond)N.

    You can say that Ionic bonds are stronger than covalent bonds, but ionic compounds are easy to break, as their source of strength (coulombic forces) tends to break them immediately if they suffer some deformation under external force.

    On the other hand, a network covalent structure is very hard to break unlike ionic bonds. Diamond, SiC, BN, etc are really hard substances. However, if one is to talk about their individual bonding, yes, single covalent bonds are weaker than ionic bonds.

    Double and Triple covalent bonds and more stronger co-ordinate bonds may be more stronger than ionic bonds. Triple Covalent bonds like N---N, or 2-covalent-1-coordinate bond in CO, they are the strongest bonds observed in nature.
     
  5. people get the misconception from high school books that ionic bonds are stronger than covalent bonds because covalently bonded substances generally melt/boil at lower temperatures than ionic. nothing could be further from the truth.

    high school books mix up intermolecular forces and intramolecular forces. as an intermolecular force, ionic bonds are pretty damn strong while the molecules bound by covalent bonds are discrete units and have weak dispersion or dipole-dipole intermolecular forces.

    However, when you look at substances whose intermolecular forces are covalent bonds such as say... carbon nanotubes, we all know that the covalent bonds are the strongest by far.
     
  6. AGNuke

    AGNuke 456
    Gold Member

    Not individual. What makes them powerful is their unity. To break one Carbon atom off the Nanotube, you need to break 4 covalent bonds, (3 sigma and 1 pi if I am correct) and Sigma bonds are not easy to break, let alone 3.

    Ionic bonds are stronger due to Coulombic attractions between cation and anion. But if we shift the lattice along a line by applying stress so that cation comes nearer cation opposite side of that line, and anion opposite to anion and so on, we see that the crystal instantly breaks along that line due to equally strong coulombic repulsion. This type of weakness is not observed in covalent structures.

    C-C bonds in Carbon allotropy are not strong, the allotrope is strong. Diamond, Graphite, Nanotubes, Graphene are not strong due to covalent bonds being strong, its because of extensive networking which united makes them strong.

    The strongest bond found in nature, between C and O in Carbon Monoxide (Bond enthalpy > 1000 KJ/mol) is due to 2 Covalent and 1 Co-ordinate (Stronger than Covalent) bonds. Conclusion, each individual covalent bond is weaker than ionic bond, but united, they are stronger than ionic bonds.
     
  7. Thanks a lot i got my answer! Bt may i ask further? If we are using this concept to explain the bond energy of HF and HI, then instead of explaining in term of bond length, can we say that the more polar covalent bond of HF lead to higher ionic character and so a greater bond strength?? Thanks a lot.
     
  8. you can't compare it like that. in a typical ceramic unit cell the anions and cations have 6-12 nearest neighbors so you can't count 1 covalent bond vs. 1 ionic bond. also there's repulsion from next nearest-neighbors. it is hard to compare condensed phase interactions with non-condensed phase interactions so all you can do is say, ok, the average binding energy is proportional to the melting point. turns out most high melting point materials are covalently bonded.
     
  9. DrDu

    DrDu 4,460
    Science Advisor

    This discussion is mostly comparing apples and pears.
    E.g. the bond energies of Na_2 is 0.72 eV, of Cl_2 2.41 eV but that of NaCl (diatomar in gas phase) 5.37 eV. So the ionic bond is stronger than the covalent bonds of the elements.
    Obviously there are substances with stronger covalent bonds than the ionic bond in NaCl.
    On the other hand there are ionic solids like MgO with extremely high enthalpies of formation.
    If ionic bonds were generally weaker than covalent bonds or the other way round, chemistry would less interesting.
     
    gracy likes this.
  10. Clearly bond strengths vary in particular situations. It is possible to conceive of a situation where ionic bonds are more difficult to break than covalent bonds.

    But you absolutely can compare covalent and ionic bonds 1 by 1. A single ion pair (or "single") ionic bond can be observed in protein folding interactions in which a single + side chain ionic bonds to a single - side chain. This bond is MUCH weaker than nearly any covalent bond.

    You can also compare the bonds within lattices. Most ionic things are in lattices and have a stronger IMF situation as a result (hence the misconception that an ionic bond is stronger - an item commonly found on chemistry exams, even). When covalent things are in lattices (i.e. diamond) they are stronger than ionic things in a lattice.

    Though DrDu's comment makes me wonder. I'm not very familiar with gaseous ionic compounds. It sounds like this might be another way of assessing a "single" ionic bond altogether.
     
  11. DrDu

    DrDu 4,460
    Science Advisor

    I just wanted to mention the definition of Paulings electronegativity scale:
    http://en.wikipedia.org/wiki/Electronegativity
    This is a measure of how much stronger a -maybe only partially - ionic bond between two atoms A and B is in comparison to the average homoatomic bonds in AA and BB, respectively.
     
Know someone interested in this topic? Share this thead via email, Google+, Twitter, or Facebook

Have something to add?

0
Draft saved Draft deleted